Question

a bike running on a high speed when applied front brake. back wheel jumps, why?

Answer 1

because of high momentum

Answer 2

the bike wants to remain moving forwards due to momentum. so when stopping the front wheel the momentum keeps moving forwards by pivoting on the front wheel's axis. this is why mountain bikers slowly move into a stop.

Answer 3

The breaking produces a net torque on the front wheel. So there is a weight shift to the front wheel. If the vehicle is light enough (as in a bicycle), it may flip over.