Question

when a fisherman jumps into the boat why does the boat move in backward direction

Answer 1

because boat mouth is backward direction

Answer 2

The answer to your question sounds tricky at first, but it's not a sailing question at all. It's a physics question.

Do you recall, that for every action there is an equal yet opposite reaction ?

Yes. You answered your own question !

The force of the person, or “Tobie Lee”in this demonstration jumps away from the boat. This forward pushing force causes an equal and opposite reaction. Hereby causing the boat to move against this pushing. Causing the boat to move backwards.