From what you descried I wonder there are 2 different type of this issue.
Type1: Only Fan speed screwed. In this case user still have the ability to set it back by software like wattman.
Type2: Driver crash. Although it seems Windows still working but no way to stop the Fan until reboot. Sometimes if one launched another GPU tweaking APP the display driver will just give a BSOD.
I alerted Fan's target speed from wattman to 2800RPM instead of default factory setting which is 2400 RPM. When type2 issue occurred it's speed is 3XXX RPM. Type2 is more like a display driver crash although you still can do operate on Windows but no way to stop the Fan speed.
I do not know what type will happen, it is randomly. but if I use DXVA enabled player(MPC-HC+LAV etc.)+game+msi afterburner in background then type2 happens more often from my experience.
I still have to say If I do not use DXVA, it seems works fine even I use msi afterburner whole day in background.
It has something to do with pixel shader when watching a video. When I watch a movie with sharpen image fan bugs out after some time, if I dont sharpen the image then I have no issues. I see a lot of people are also complaining about decoding not working properly so i assume it has something to with that.