Here's my opinion on the matter.
Rampage has battered Chuck twice, once quite recently by vicious KO. He seems to be improving constantly.
Chuck won a close fight against Wandy quite recently. Wandy's abilities seem to be fading away.
Rampage's win over Wandy, particularly the fashion in which he won puts him right in the thick of the title picture, if Wandy won then it would have had the same effect.
Chuck wants the title and the 2 main guys above him have viciosly knocked him out, whereas if Wandy one it would likely set up a number one contenders match between Chuck and Wandy, a fight he would feel he could win. It does'nt bode well for his title dreams.
I believe when Chuck saw Wandy drop like a sack of spuds he thought "I'm never going to get my title back" thus the look of sadness and dissappointment draped across his face.