Duty cycle limits are normally designed to prevent a motor from overheating. Not sure why exceeding them would cause a breaker to trip.
In fact, this is possible with certain types of DC motors. Overheating may cause permanent magnets to lose some of their strength (way before reaching Curie point when magnets turn into a paperweights), which may cause motor to stall under less than rated load - and then overcurrent protection device will trip, be it a circuit breaker or something else. I've observed this many times while doing servo drive programming. Normally, a thermal protection - if it exists - should kick in earlier, but under certain conditions it is perfectly possible.