Pork shoulders do that sometimes.. we had a case not too long back where someone smoked their pork butts for 22 hours, at the 22nd hour if memory serves their temps stared going down.
My personal opinion is there's simply more connective tissues melting down, which might be bringing it back down in temp.
But with pork butts.. nothing is for sure. It's simply done when it's done.
Shame those sammies taste so good, eh?
The Life of the Knife is Ended By! The Wife.