Well, to solve your problem you are going to have to do some tests. Are you set up to do basic rf testing? Did you design the amplifier in quetion, or are you just buying it?
It is likely that it is oscillating, off frequency, during this test. Since oscillations are going to drive the amplifier HEAVILY into compression, there can be big gate currents flowing, which will damage the output device.
You can set up the amplifier with a broadband directional coupler on the output, a 1 dB high power pad, and a moving short circuit. Vary the length of the short circuit, and look at the spectrum analyzer on the coupled (and appropriately attenuated) arm, and see if it breaks into oscillations. Especially look at things like 200 MHz, etc.