Butter is slippery and when you're trying to add a lot of it into a well-kneaded dough with a strong gluten network, the first thing it does is coat the entire outside of the dough. This makes it quite slippery and the dough hook might have trouble picking it up. Usually if you just let it go it will snag the dough again on its own, but if it's really having trouble you can pause the mixer and use a spatula or your hands to fold the dough around the hook again.