I had my first (that I can recall) reset of Heavensward last night and I have a very, very stable system so it was a shock. I even wondered if I'd had a momentary power cut (I hadn't). Complete system shutdown as if it had overheated but my system is heavily monitored (ROG kit) and temperatures were very low compared to most people.

I have no idea what causes it. I was just in Foundation after doing some random stuff and was about to change gear and "poof!".