My guess is that it is open transition (break before make), and the two sources are not synchronized. Since during re-transfer back to utility, both sources are available, the re-transfer happens very quickly and the motor ends up with a tremendous inrush due to out of phase transfer. That may be a cause of starter lock-out due to a protective safety setting in the starter. Some transfer switches have a delayed transfer or an in-phase monitor, where either the field in the motor is permitted to decay prior to transfer or the transfer is only permitted when the "good" source is in phase with the load (decaying field).
