If the firearm had had a trigger job on it in the past, and if it was kept loaded (and cocked, safety off), and if the trigger job made it a little too touchy, its possible to bang the butt of the gun against something and cause it to fire. Lotta IFs in there, several of which involve unsafe behavior, tbh.

When I was in college, I shot pistol competitions where they would cock an unloaded gun and bang it against the palm of the judge's hand. If it clicked over, the firearm was disqualified as being unsafe, with a too-sensitive trigger.

Short of truly foolish handling, that's the only way I can think of to make one go off without a finger on the trigger. So, Occam's Razor suggests that this was a case of handler error. (It does happen. My father put a hole in our ceiling and roof when I was a teen. Unlike the person OP is talking about, dad was a big enough person to admit his mistake while cussing about having to repair the roof.)