Other than someone actually turning the statue, how else could this be happening?
The reasoning that "it has been there for decades and hasn't turned before" doesn't really hold water for me. Anyone who has owned a home know buildings shift and settle all the time as well as mounts, shelves, and everything else losing a bit of structural integrity over time. It only takes a screw or two to get a wee bit loose. At least that is my simpleton view of it.
It's a stone statue over a glass shelf, so the friction coefficient is probably low and it's possible that the vibrations make it turn around.
Perhaps before the shelf was inclined a few millimeters to the other side, so it didn't turn.
Perhaps it was formerly in a wood shelf, with more irregularities and a bigger friction coefficient.
Perhaps there was no camera, so when someone found that it was looking to the other side, they just assumed than someone else had moved it.