From my own research, I only ever found reference to them falling over naturally due to very shallow holes, as already stated.

Personally, I wouldn't want then re-erected as there is no guarantee they are being re-placed in their correct positions and Arbor Low is still very impressive as it is. In some cases there is an argument for restoration/reconstruction when a place is under threat, but don't see that as being the case here, so my vote is to let sleeping stones lie.

A.