All acts of vandalism are just mindless and irresponsible but some defy any sort of explanation whatsoever.
Look closely to the right of the seat and you can see a patch of wall that has been repaired. I reported this to the National Trust just last week and they have promptly been up and repaired it, probably on Friday or even yesterday.
I can’t see clambering sheep being the culprits. To push a wall over would take some doing without mechanical assistance — a motorbike or quad bike perhaps.