Exterior mirrors rotary knob

Rotary knob of external mirrors felt inside today and won't sit firmly again.

I don't know why, didn't touched it for a long time.
Just closed the door, went home and after came back saw this.

21 800 km, 1Y3M

Update (2012-03-30)
Knob fixed. You need first insert left side and after pull until right side enters housing.
But still not understand why that happend :-)