It is either a bad pump, a short in the wiring, or a bad breaker. To test, you need to disconnect the wires to the pump as close to it as possible. Flip the switch. Nothing should happen and the breaker should not pop.
If that is the case, there is no short in the wiring.
To bypass a bad breaker, you have to wire the pump directly to a power source. That can be done at the pump wires or at the breaker by disconnecting the wire at the breaker and temporarily moving it to another breaker and then testing.
Although breakers do go bad, it is more likely that you have a short or a bad pump.