Question

why does a back seater move forward when a fast moving bike is stopped suddenly?

Answer 1

because the seater still inherent the motion of the bike which is called law of inertia

Answer 2

Due to property of inertia, as initially the body was in motion but when suddenly the fast moving bike stopped, inertia of the body of back seater opposed the change and he moved slightly forward.
Hope this will be helpful :)