I'm guessing here but:
When you lower flaps in a C172 you get a heavy downwash on the elevator causing the plane to pitch up. (just try flying straight and level and apply flaps)...
When you sideslip, some of this downforce might miss the elevators because you are flying the plane sideways into the wind. Also the tail fin will block some of the downfoce - again because you are flying sideways. All this will happen quite turbulent, and will cause the downforce on the elevator to vary quite a bit, causing all sorts of pitch issues.
Again - just a guess..