I really don't know why a stall happens, I just know it does. I'm not a scientific guy or any kind of trained chef, but I do have an opinion. I think when a butt reaches about 160-165*, the heat is absorbed by the breaking down of the connective tissue and collagens. When that process is completed, the heat returns to the meat. I think that's what I've read. I don't have any way of being knowledgable other than that. If someone can give a better explanation, I'm ready to read some more.
I hope this gives you some help, Dave. Good smokin', Joe