Question

why does a fielder pull his arms back while trying to stop (or) catch the 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

Because when the fielder pull his arm back it reduces the momentum of ball due to which the ball exerts less force on the arm of fielder.