Regarding your original question -- are we talking actual, true wetness, or do you think it is a subjective "feeling" of wetness, based on the fact that your butt suddenly feels colder? If sitting down on cold metal seats squishes the air out of the insulation surrounding your buttocks/thighs area, wouldn't that produce a similar sensation? Or maybe it truly is the condensation issue that someone mentioned.
I know that when I go inside and take my snowpants down in the restroom, I feel this almost instantaneous "Whoosh" of subjective cold, as all the wicking layers suddenly hit the warm air and instantly do the evaporation thing. I then find that there is significant condensation on the inside of the snowpants. Perhaps it is possible that the condensation builds up in the butt area, but you don't feel it until you actually sit down in it???