Temp plateau on large chunks of meat

Evaporative cooling. The meat sweats and the convection within the cooker cools it. Just like when you sweat, the wind cools you. Once the meat quits putting out so much moisture it breaks through the stall.

http://www.genuineideas.com/ArticlesIndex/stallbbq.html

http://www.huffingtonpost.com/craig-goldwyn/physicist-cracks-bbq-mystery_b_987719.html