Depending on which "Breaker" you mean:

1. There is the ACTUAL breaker at the panel in your house.
If this is the case, your pump is overloading the circuit, which it probably should not be doing if it's rated for that circuit - suspect problem - pump is requiring too much power to start.

2. There is the GFCI which is an outlet (plug) with a couple of small buttons - test and reset between the outlets.
If this is the case, it can be one of two things - an actual ground fault, but every time you turn the pump on, it would immediately blow the GFCI if this is the problem.
Number 2 problem - the GFCI is dying - a common issue, replace GFCI outlet and things should be OK. Also, GFCI's can act up with high humidity but this is also a sign of it failing.

3. Sometimes there is a breaker on the pump, if this si the problem, then it is internal to the pump and your pump is at fault.