I've seen this sort of configuration before in motor drives using IGBTs. The diodes (usually TVS devices) are there to prevent peak switch voltage from exceeding some level, which would cause failure. When the voltage rises higher than the reverse breakdown of the diode, then the transistor will turn on regardless of what the controller is doing. The reasoning behind this is that it's better to have a momentary turn on than to let the switch die, which would likely result in it becoming a permanent short circuit.