Not only rocks, but also often while proning and aiming downwards.
See: http://feedback.dayzgame.com/view.php?id=5230

I have also had this issue while in prone position.
In my case i was aiming downwards with clear line of sight infornt of the barrel.
Pristine weapon, attachments and ammo. Bipod deployed and long range scope used.

Location: Upper balota hill near treeline (Although i think the location is irrelevant)