frozen
My sled was doing this on monday morning, it was particularly cold compared to the rest of the season, and I had been going through alot of deep snow the day before. I was suspecting some snow getting to the carbs through the airbox and melting, then hanging out until the sled cooled down when it froze the slides. I could open the throttle more but the slides would not drop back down, and my cable was actually disconnecting from the throttle lever.
Later in the day when it warmed up I was able to get it all freed up pretty easily. When I pulled the caps on the top of the carbs there was a little bit of ice, like about 2 frozen drops of water sitting atop each of the slides. Works fine now, I just oiled up the cable to be sure (my Clymer shop manual actually calls for a weekly oiling of the throttle cable to prevent any sticking whatsoever).