Alternator

Terminal voltage exceeds generated voltage?

Low generating frequency is also one of the reason for motoring action.

Other parallel alternators producing a higher voltage than the alternator in question, thereby causing current flow into it (rather than out of it) and thus making it a motor.

Uh-oh, common misconception here! Unlike DC machines, raising the voltage on the terminals of a grid connected alternator only changes the VAR flow, not the Watt flow (ok, a little bit to overcome the change in copper losses, usually ignored though).

An AC alternator "motors" because the prime mover is no longer capable of putting enough torque out to overcome the mechanical losses plus the electrical load on the generator; i.e., power (Watts) is flowing into the alternator instead of out of it.

In power engineering terms the alternator's power/torque angle (not to be confused with the power factor angle) is lagging behind the grid's.

btw- it's a bad situation for an alternator to become a motor since it spins the turbine with inadequate steam flow to cool the blade tips, or a diesel engine is receiving input power on its output shaft.

It is having an identity crisis. It is confused about its real function.

Identity Crisis - DC