[greglepore]

Everyone here covered it mostly. GFI breakers tend to age/fail more frequently than regular breakers. It could also be a failing motor. The thermal limit should trip at the motor before the breaker trips but I've seen them where after the limiter cools enough to restore power the bearing itself is still hot enough to present enough extra start load to trip the breaker. You wouldn't perceive this as "trip on start" unless you knew the motor was heating enough to trip the limiter.