Sometimes breakers just fail due to age or inferior quality. If the problem does not appear to be with your pump and/or motor, try replacing the breaker.You could also have the wrong size breaker if you recently bought a new pump or motor that was not an exact match to the previous model. Confirm you have the correct breaker size by checking how many amps the new motor requires.
You might be able spot an obvious pump motor short by disconnecting power and taking off the motor end cap. Make sure all wiring is properly connected and nothing appears obstructed or burnt. Insects who decide your motor makes a really cozy home can create an electrical short by blocking contacts. Other possiblities are a seized motor or defective coil insulation due to age or short. When a seized motor turns, it draws more amperage, which makes the breaker to trip.
Most complete pumps and replacement motors leave the factory set at 230v to prevent installers from accidentally running 230 volts to a motor that is set to 115V. This has saved many motors from a premature frying but it also results in many confused homeowners. Typically in this scenario, the pump will initially come on, then shut down, repeat. Check to see which voltage your motor is set to and what voltage you have running to it.