Butter Tablet Eruption

Note that the recipe refers to reaching a honey caramel color before beating, and the final color looks to be a bit lighter than that, so the darkening during beating does seem like a bad sign.

So it sounds like, before the eruption, you had some continued caramelization during beating. It's a bit of a guess, but possibly your pot was heavy enough to retain enough heat to keep on cooking after removing from the heat. In that case, as the beating thickened it, it might've eventually gotten to where instead of bubbling a bit, it held on to all those bubbles and foamed up.

To address that, I might try transferring to a bowl to beat in, or letting it cool a little bit before beating. I know that for maple fudge, which is a pretty similar texture, you actually do both.

I could also see weird things happening if you got the ingredients wrong somehow, and for example had more liquid than intended, so too much was still left to boil at the end. So it could be worth double-checking quantities, but given that you tried it twice and it's a simple recipe, I assume that's unlikely.