Why is reverse power protection required in units like signal generators?

Results of Motorization of a Generator

If a generator loses prime mover power, it acts as a motor with a DC field on the rotor

The DC field will cause the rotor to try and follow the AC field in the same direction as before

If the mechanical drag on the rotor is heavy, it will fall behind and "slip poles,"inducing a large voltage into the rotor; this can cause insulation breakdown of the windings, flashover at the brushes, and violent shaking of the generator mountings.

If a generator loses DC excitation to the rotor it will not generate, but the prime mover power will still turn the rotor

Now the generator acts as a motor running at no load