Water pump connected to regular electricity connection trips after 5 minutes on its switch on. It never trips if connected to a diesel generator. Possible reasons?

Guessing from what little information I have, the line power circuit breaker is a lower amperage than the one on the generator. The fact that the motor runs on the line power for a few minutes indicates to me that the motor is running near the capacity of the breaker and it takes a few minutes to trip or the line breaker has additional loads on it that the generator doesn't and the accumulation of loads on the line breaker are above the 80% capacity of the breaker and it trips in a few minutes. There could be other issues, but this sounds most likely to me.