There is no soft mechanism on a nail gun, per se. The fact these were 3"+ nails indicates this was a framing nailer.
The only safety on a nail gun is in the nose of the nail gun. In order to fire the gun, you need to physically press the nose of the gun onto the work surface then pull the trigger OR vice versa [pull the trigger and then let the nose press do the firing].
Nail guns are coil or clip fed full-automatics. There is a peculiar behavior of nailguns in that when you fire them, the recoil tends to bounce the gun up an inch or so from the work surface, and then when the guns weight carries the nose back down, the gun will fire again if the trigger is still pressed.
Its not uncommon at all for the gun to double or triple tap before you even know what happened.
Having used nail guns for years, and employed people who have used them, and have shot themselves...here is how I think this happened.
During the fall, the panicked worker holding the nailgun kept his finger on the trigger [as its normal to do that], and the gun just did a couple of quick triple taps of this guys noggin...OR...the worker with the nailgun somehow got tangled up with this guy after the fall, and he was under the victim, with his hand on the trigger.
As bizarre as this story may sound to some freepers, I don't at all find it that strange or unusual.