I’ll bet that the “1/2 Bank” option on your autopilot is getting mysteriously activated. This occasionally happens to me… on a somewhat complex STARS route, the plane kept trying unsuccessfully to keep up with the turns. Something activated the 1/2 bank option. The top of the PFD Attitude Indicator will show a small arc near the slip/skid indicator if it’s activated. I know I’m not accidentally clicking it.
I posted this in a separate thread, with no real answers.