He got outstruck from range by a former undefeated world champion kickboxer. I think a lot of people underestimated Thompson as still the one dimensional striker who got mauled by Matt Brown nearly three years ago. Hendricks couldn't get inside Thompson's range to work his superior wrestling, the longer the fight stayed standing, the more it favoured Thompson. This fight was nowhere near the one sided grinding beatdown that a lot of the 'experts' predicted.
Fighting at this level, it's easy to 'fall off a cliff' as you age, lose motivation, acculmulate damage etc. Look at someone like Gray Maynard, could have and probably should have beaten Frankie Edgar for the LW belt, lost the rematch, squeaked a decision over a rapidly declining Clay Guida, got easily finished twice and then dropped a decision to someone coming off a decision loss to Nico Musoke.