My guess is 'b'. Happened to me twice in quick succession with my gripped 5DII, the first time I thought maybe 'a' but not twice. It happened when I added the 2xII behind the 70-200 II, and that changed the center of mass of the combo such that the lens release was pointing slightly upwards, and apparently hit the belt loop of my jeans (reinforced by the leather belt underneath it). Fortunately, the camera was cosmetically undamaged despite a fall from waist level to pavement, twice! It functioned perfectly fine, with one exception – all of my AFMA values shifted by about 10 units to the negative. Something to check after drop, even if everything seems okay.
That happened when I was still using Manfrotto plates, and the Blackrapid strap was connected to a FastenR-T1 through the RC2 plate on the lens collar. Since switching over to the Arca Swiss system, the Blackrapid lug is attached to a small 1" clamp with Loctite, and I can adjust the position of the clamp along the lens plate to balance the rig appropriately – there has not been a reoccurrence.