Door mirror self adjusting.

Hi, I was driving down a motorway today and I noticed that the drivers side door mirror had adjusted to the right, I had it set so you could just to say see the side of the car and it had moved so you could see about 2 foot away at the back of the car. This is the second time it has happened, it did it about 2 months ago, exact same thing. I don't know what has caused this and it isn't a major issue as far as I'm aware, the stalk still moves the mirror and can just be adjusted back (and no, I didn't touch the stalk or mirror) Just wondering if anyone else has experienced this and knows how to rectify it.