Are you also hitting the brakes? Rear wheels are pretty easy to lock up with adequate brakes.
Not sure what you mean by "skidding." Either you have no air in the thing, or are taking corners very rapidly. I suppose it's possible that the tread pattern could be that knobby, but even so I have to be going pretty fast around corners to have even a MTB tire lose grip, and then it's usually both tires at the same time which always causes a nasty fall.