(YouTube link)
We previously showed you video from the Ueno Zoo in Tokyo, where they use fake animals to train for escapes. The latest drill features a rhinoceros! An escaped rhinoceros? They should have used a bulldozer instead of two guys in a rhino suit! -via Arbroath
I think they may have realized the consequence of stealing ones bucket after the Walrus fiasco.
So the rhino goes on a rampage because someone stole his bucket.