It could be several things, but the number one reason for a spa to have intermittent breaker trips is a bad heater.. In the beginning the heater works, but has an internal high resistance short to the water, popping the breaker.. This problem tends to be intermittent at first.
I suggest that you disconnect the heater and run the spa and motors and see if you still have the problem.
How long does it run before tripping? Nuisance tripping can be caused by all kinds of things, including the heater as Jim said. How long it runs and what it's doing when it trips can be key to figuring out the issue.
Typically, if it runs for hours to days between trips it is a circuit breaker issue. Maybe a bad breaker, maybe loose connections getting hot. If you disconnect the heater it may stop tripping, but not because the heater is bad. The heater is the highest amp draw in your spa. If you eliminate that current draw it may not cause the faulty breaker to get hot and trip. If it is the breaker it will feel warm to the touch when it trips. It should not be any hotter than the box it is in.
Might also be an ozonator (if equipped) cycling on since it ran from, assuming fresh fill, 50-ish to 88 degrees before tripping. I would unplug that first. If you are friendly with an electrician, a megohmeter or megger is the penultimate troubleshooting tool for GFCI issues. Just don't touch the water during test!!