Could be all sorts of things, could be the picture just fell off the wall as a result of the picture hook failing, string fraying, wire rusting through...
There may have been no banging noise but she misremembers one in trying to explain why the picture fell off, could be it falling off gave her a big shock..
Could be she has hearing issues, dementia, drinking problems, other issues that cause her to hear things that are not happening - my mother was CONVINCED, to the point of notes in the window and calling the police, that someone was banging on her window at 3am several nights a week, every week. We set up a camera... on the nights she claimed it happened, there was no one there!
Yet despite telling her this and a few times, staying over in the living room (same side of house, window out onto the garden), she still claimed it was happening and that it was escalating to faces pressed up against the glass.
She was seeing and hearing things as a result of her drinking problems. Nothing else.
Just politely decline, if anyone shows up asking say you're very sorry shes upset but there was no one in the room at the time she originally claimed it happened nor for many hours after and it is simply not possible so you won't be paying.
She has no legal leg to stand on so I wouldn't worry about it too much!