I have known some hunters to 'leg' their shot rabbits & hang them on the fence, to be picked up on the way home, so the fleas jump off in the field rather than inside your 4x4. I have also heard of the Gibbet or vermin line, but those seemed to be on fences where passing people of importance (the squire, landowner, gamekeeper etc.. ) would see them. All of those were good old country traditions with a sound reason for doing it.
But leaving a fox in a tree? That sounds daft & may be the work of someone up to no good, such as teenagers or the local louts... No good reason for doing it. The fact that it sounds like it had not been shot is interesting. Could have died been hit by a car & thrown into the tree by the impact (if near a road).