Question

when any block slides down a smooth incline which is unfixed then the incline moves back.WHY??

Answer 1

This will only be true if there is low friction between the inclined ramp and the ground surface. (For example, the ramp might be on rollers or tue surface below might be ice.)

The motion is due to the reaction force on the ramp.

Answer 2

Conservation of momentum. The ramp will move back with equal but opposite momentum from the block. In any case, even a fixed ramp, momentum is conserved but it becomes a much larger system, the earth ramp block system.