Who originated the dangerous avocado-pitting technique?

I sincerely doubt any particular chef or "popular show business personality" is responsible for this, though perhaps one or more such people have done this at one time or another.

Many people are used to taking other fruits in one hand and a knife in the other and preparing the fruit for eating. I distinctly remember my father showing me how to core an apple with a pocket knife when I was about five years old. Yes, I believe there was a warning about not stabbing yourself with the point, but people commonly use paring knives all the time with fruit held in your hand to peel things, take unedible bits off (as in hulling strawberries), and sometimes to remove pits and seeds.

I think many people who are unfamiliar with avocados attempt similar maneuvers. However, the avocado pit is much larger and more difficult to dislodge with a knife than with many small fruits. There is also a smaller proportional distance between the pit and the surface of the fruit than many other fruits (again due to its size). Simultaneously, the avocado flesh is much softer than many fruits people are used to dealing with, thereby making it much easier for a knife to accidentally slip completely through it than with, say, an apple when coring. Finally, the skin of an avocado is what I'd call "deceptively hard": when you try to slice into an avocado, it may seem quite firm compared to the interior, but it's quite easy for a knife point to slip through the skin from the interior to the outside (particularly when one is applying leverage to try to dislodge a large pit).

So, you have people doing similar things with a knife to what they might do with an apple or a peach, except they end up exerting more force for the large pit while the knife slips through the flesh like butter, and the knife goes awry.

I don't think it's a great mystery why this happens. One might equivalently ask why bagel slicing sends people to emergency rooms. (It's one of the most common pathways from the kitchen to the ER.)

EDIT: Well, I've been downvoted -- not sure why, but I'm going to assume because I didn't cite sources. So, I'll try to bolster my point. (Pun intended.) Here you go:

A recent study estimated approximately 8900 avocado-related hospital visits in 2018. The study notes: "The oldest person to visit the ER for an avocado-related laceration was 75, and the youngest was 8. Most of those hurt by avocados were, in fact, millennials." So, there's a wide range of ages reported, making it seem less likely that all such injuries occurred because of bad instruction from a single source. I've skimmed at least a dozen articles on avocado-related injuries and have not seen a single mention of any celebrity chef or other "popular show business personality" as the source of the bad knife technique. (To the contrary, many articles cite authorities who give better techniques.) And then you have an explanation offered by a doctor in this article regarding bagel injuries:

Maybe it's just their shape. "Any spherical food, like tomatoes and onions, and particularly bagels, people tend to hold and cut in their hand," Dr. Smith-Coggins says.

Indeed, a decade ago, in another study the top five foods related to knife/finger injuries were (in order): chicken, potato, apple, onion, and bagel, each of which also results in thousands of serious injuries. As avocado consumption has increased about five-fold in the U.S. in the past couple decades, it seems they're the new spherical food causing injuries.

As a final note, it is commonly recommended by many chefs to use the blade of a knife to remove an avocado pit (as here, for example, which also shows the somewhat dangerous practice of scoring the avocado in your hand after depitting). This pit technique apparently does cause some injuries, particularly as people struggle to get the pit off of the blade of their knife. (The more I read about the stuff, the more weird ways it seems people could get injured dealing with avocados, so I'm not sure "avocado hand" is caused by only one type of mishandling.)

In any case, I have yet to find a similar example of someone famous telling people to stab the pit with the point of a knife while holding it in one's hand.

So to answer the question, James Peterson recommends the technique in his 1998 book Vegetables. His books are used as texts in culinary schools and are also read by cookbook authors, so it's not impossible that he's responsible:

To prepare avocados, just cut them in half lengthwise with a chef's knife---rotate the avocado so you cut up to and all around the pit---and pull the two halves apart. To remove the pit, carefully give it a whack with the knife blade so the blade embeds itself about a half-inch into the pit. Then twist the knife to get the pit to pop right out.

Rick Bayless' 1996 book Rick Bayless's Mexican Kitchen suggests the same technique (another suspect).

The alternate method, which you'll see in books printed in Mexico and also Dianne Kennedy's (1989) The Art of Mexican Cooking involves peeling the skin off with a knife and then pulling the flesh away from the pit (or gently mashing the peeled avocado together with the pit). Here is Kennedy's advice:

You never really need to peel an avocado whole, which is a messy business anyway. Cut it open, remove the pit, and scoop out the flesh with a wooden spoon ... To slice an avocado, cut through the skin and flesh down to the pit with a sharp paring knife, then pull the skin back; with it a section of the avocado should come cleanly away from the pit.

Her comments are rather interesting. If she advises against peeling the avocado whole, it means that that's how she saw people doing it in Mexico. Also the wooden spoon is something that doesn't seem current in the States. So one wonders if the knife technique is current in Mexico...

Yolanda Ramos Galicia's (1993) Así se come en Tlaxcala, gives a few traditional recipes involving avocados that she took from interviewing locals in Tlaxcala:

Los aguacates ... se lavan muy bien y se escurren ... los aguacates se pelan, se les quita el hueso y se deshacen en una cazuelita con una cuchara de palo y un poco de agua.

Se lavan muy bien los aguacates. Se pelan y en un molcajete se deshacen con el tejolote.

Se lavan muy bien los aguacates y se pelan. Después en una cazuelita honda se machacan con un tenedor hasta que se deshagan bien.

Aparte se pelan los aguacates y en una cazuela se deshacen con la mano.

[The avocados are washed well and dried, the avocados are peeled, the pit is removed, and they are mashed with a wooden spoon and a little water ... Wash the avocados well. Peel them and put them in a molcajete mash them with the pestle ... Wash the avocados very well and peel them. Then in a deep dish mash them with a fork until smooth ... Peel the avocados and mash them with your hands in a separate dish.]

In Mexico traditionally cooks would use a dull knife to cut vegetables in one's hand, and molcajete and metate for other prep work. In the US very much is accomplished with a cutting board and a heavy, sharp knife. Also in Mexico they may use varieties other than Hass that have thinner skin (which better lend themselves to peeling with a knife), and it's believed that leaving the pit with the flesh slows the oxidation (so the pit is left in the bowl while the flesh is mashed around it). The wooden spoon concept is probably also to discourage oxidation, or maybe because there's always a wooden spoon at hand and it's a good blunt instrument. Finally, the recipes insist on washing the avocados, which is especially important if the skin is peeled off with a knife.

All this suggests that the technique with the knife for removing the pit originated in the US rather than Mexico, since the technique would have only occurred to someone working on a cutting board with a chef's knife. Two popular books from the 1990's advise the technique.

Finally, the technique isn't dangerous unless done improperly. It is the blade of the knife, not the point, first of all. You might be tempted to do that if your knife is dull (another warning sign). Working with a chef's knife is inherently dangerous if you haven't had a basic safety orientation, and I bet that would explain most of the accidents.