Funny this question came up because I just had that happen after class this morning (Interval Max). But it is not a new occurance for me. I find that this happens to me when I am working out exceptionally hard. After my workout I have this ear popping internal moisture type feeling that affects my hearing for about 15 minutes(sort of the way your ears "feel" after you have stepped out of a room where the music was blaring).
I have tried to get the answer to this question too. I asked an ENT once and got an answer that basically said it could be due to many variables (ie: working out in high altitude, working out with a cold, working out in air conditioning etc.). While I never got a direct answer I'm guessing that it has something to do with the sinus area too.
Sorry that I don't have an answer for you but wanted to let you know you are not alone.