This happened to me before, I think more than a year ago in both mirrors. Now it happened to me on the driver side mirror.

I noticed it happened after a car-wash with a power wash. Perhaps the power-wash slowly removes lubricants from within the mirror. After some time the mirror starts closing only partially and it really feels like the engine isn't powerful enough to overcome some friction in the mirror.

After finding the solution on youtube, I tried it and it worked for me. Still, I'd expect it to not happen at all.