I have had that happen due to RF feedback. It wouldn't happen at low power, but at high power the rig would just shut off. I suspect the RF was getting into the CPU and crashing it. This can happen when the antenna is too close to the rig or when you have high common mode currents on the feedline. In my case I resolved it by moving the antenna farther away. What sort of antenna system? How far is the radiator from the rig? I've owned several Yaesu mobiles and I've never seen a one shut itself off due to high SWR, even when transmitting with no antenna connected. But I can't say for certain that it never happens.