Why do some eggs crack while being boiled?

Well, sometimes eggs have non-visible (or non-visible without candling - shining a very bright light into the egg and looking at the light coming out of the egg) cracks.

That would be one possible cause - the crack pre-existed. Even if it was not cracked when graded, cracks both visible and invisible are possible in the distribution process.

Getting cracked from moving around in the pot is possible, depending on how hard you boil them. If actually boiling them I'd suggest a simmer, but in fact I don't suggest that at all - just boil the pot of water, lower eggs in with a spoon, turn the heat off and cover - yank when they suit your concept of "done" (if hard boiled, 20-30 minutes by this method.) "Coddling" is sometimes used to describe this, though it's also used to describe cooking (without shell, and possibly with added ingredients) in a specialized ceramic container (an egg coddler.) Anyway, it greatly reduces the odds of cracking via "rattling around in the pot" (though not why I do it that way; I do it because it appears, observationally, to reduce my incidence of overcooking the yolk and making it nasty.)

The thickness of the egg shell varies. The cooking procedure results in a pressure of about 1 bar / 14 psi inside the egg, 0.3 bar / 4 psi due to the expanding air and 0.7 bar / 10 psi due to the expanding water of the egg white - at a temperature inside of 90°C/194°F, which is a reasonable maximum temperature during cooking.

Some egg shells can bear a pressure of up to 3.5 bar / 50 psi (yes, more than a tire!), but some already fail below 1 bar / 14 psi.

(The answer assumes chicken eggs.)

I used to have the same problem. It is almost always a result of the shell being too thin and/or the crack preexisted. The crack may be a very fine line, barely noticeable to the naked eye. (If it's fine enough, the membrane just inside the shell will hold the liquid at bay.)

If the egg is cracked, it the increased pressure caused by the rising temperature as the egg cooks will almost always cause it to split open, breaking the membrane and allowing the white to begin to escape.

The solution: prick a small hole in the rounder end of the egg before cooking it. You need to do it to the rounder end because this is where there is an air pocket (I call it "headspace") between the shell and the membrane. The idea is to make a hole that goes through the shell, without puncturing the membrane. In doing so, the hole will allow the expanding egg white (and membrane) push the air out of the hole as it heats up. (You will see that happening when you submerge the eggs in hot water.)

I like to use a push pin (repurposed bulletin board tack) and hold it between thumb and forefinger in such a way that no more than about 1/8" of the pin will go into the egg.

I boil about a dozen eggs every week and always take them straight from a 34° F fridge, poke the holes, and put them straight into boiling water. Since doing it this way, the only eggs that leaked were visibly cracked to begin with. I cannot remember the last time it happened. (I gave up experimenting with the cracked ones and use them in other ways if they're not spoiled.)

If you purchase genuine free range eggs from a farm that feeds there hens on good quality feed, the eggs never crack when boiled, supermarkets that claim free range eggs are normally kept in a barn, grossly overcrowded and to keep prices down are fed on a relatively poor diet which is why the shells are thin and weak