I suspect that failure happens when you switch off supply voltage. At this instant transistor maximum ratings are exceeded. Check bias circuit, check the behaviour of your power supply when you switch it off.
BGW425W can get +12dBm linear output power with a power gain of 17dB. That means could support -5dBm at the input without problems (if the bias is correct).
Try to match the output for maximum power and not for maximum gain, and also check for stability at a large frequency range.