I just covered this in a class I taught yesterday.
The 870 safety is a trigger block only... it is designed to enhibit the rearward movement only.
If you can get ahold of a cross section of the action you will see that the "trigger sear" is NOT holding the hammer from falling. There is a small arm/mechanism that the trigger moves/pivots (this mechanism has a small return spring to move it back into place for the hammer to be cocked again) This arm/mechanism is what releases the hammer. All I can say is that the gun has to be jarred on the correct angle.
I remember hearing about a duck hunter's wife attempting to sue Remington for Wrongful Death after he tryed to "club" a wounded duck with aloaded 870. Unsuccessful.
Be Safe, Be Trained
Life is tough...it's alot tougher if you don't know how to shoot.