Circuit Breaker Tripped Due to Turbine Under Speed

We have a dual fuel frame 5 gas turbine driving a generator (25 Mw). The circuit breaker got tripped while the turbine was running on gas fuel causing site shutdown, then it transferred automatically to liquid mode due to fuel gas compressors trip after shutdown.

-The annunciator alarm was Turbine Under speed.
-The turbine control system is SPEEDTRONIC MARK II.
-reclosed the circuit breaker again successfully.

What could be the reason of underspeed action?
The cause of underspeed when the generator breaker is closed and the unit is producing power is low frequency, which is caused by excessive load with respect to the amount of generation.

The alarm is to protect the turbine and axial compressor from operating below its design range--which can cause very bad axial compressor damage.