Question

why does a cricketer move his hands back ward while catching a fast moving cricket ball?

Answer 1

A Cricketer moves his hands backwards during catching so as to take a longer time to stop the ball,this reduces the chances of injury.
According to Newton’s law of motion,
Force=Mass*(change in velocity\speed)/time
Greater the time used to stop the ball,lesser the impact of ball on hands and thus less chances of being hurt by ball and get injured

Answer 2

Cricketers move their hands backwards in order to not to get hurt and to use less force in order to stop  the ball with less momentum. According to second law of motion