[I] have a 50 amp GFI (about three years old) which trips infrequently - about once a week or every other week.
TJ Handyman

Check the amps when the motor starts up and when it is running, motor may be getting old and drawing to many amps. Are the breaker could be weak or defective.If motor is in the heat it could possible draw to many amps after running for a while also.

So this is on a hot tub, right? What amperage is printed on the motor for runnning and starting. Do you have it set so your heater comes on automatically, how about air jets? All of the functions running at the same time should be printed in your owners manual, 50 amps is slightly on the low side for many hot tubs.