Overspeed

Why is overspeed dangerous for the generator?

Especially with bigger systems, the generator is not rated for much more speed than the operational speed. Check your generator ratings.
If, say, the AVR (Automatic voltage regulator) or the excitation system fails, there will be no load on the turbine, and the generator will speed up. The centrifugal force will loosen the wire windings of the generator and tear them apart.