Close call

Had it happen a number of times on my D3 before I figured it out. Key ring wasn't that heavy either. The usual cause was a bumpy road with keys inserted so that the weight pulled the key around to off when a bump was encountered.
Fixed by removing some weight and inserting key so weight of ring turned key to on position, fixed completely by D4 with keyless ignition purchase.

None of my cars have anything on the key rings other than a small RM Williams key ring or a basic one with the key number, as said above can damage the barrel for the lock and so forth. But thankfully keyless go has alleviated this problem somewhat.