I've been doing this the last few flight and it hasn't happened again. Before I was just keeping the throttle up and speeding down on decent. I mean, I'm not over-speeding so I should be able to fly around 240kts on decent right? It was way north and well below freezing though. So maybe frozen pitot tube at those speeds (I had the pitot heats on) I don't know?
I had just repaired the Air frame prior to this flight because I added reverse thrust while standing still and the tail hit the ground breaking my tail trims and body. It was quite fun after I took off because I didn't know and got cabin pressure warnings above 10K the flight just before. Emergency landed and fixed it and I always check Maintenance components before starting up
I've started paying attention to this now and it hasn't happened since. Very likely as there was some extreme wind and I never re-set aileron and elevator trim before disabling autopilot. Still doesn't explain wandering off Nav Autopilot though.