Sounds as if you are locking up the rear wheel.
What kind of road conditions does this happen on? Wet, loose surface, snow and frost, old leaves all make skids much more likely. Clean tarred surfaces have the best grip, but you can still lock a rear wheel on them.
Brake lightly on the back and it probably won't happen. You should anticipate ahead of time if possible so that you brake more gently. It is especially easy to lock the rear wheel if you have decent brakes. You can usually recover from a rear wheel skid by letting off the brake and compensating with the front, but a front wheel skid though much harder to initiate is more difficult to recover from.