Yorick Ult bugged?

This could just be because we got smashed by a Yorick last game, but around three different times during the game he decided to dive us his ult tanked the tower at around 20% hp. I was planning Annie at the time and did my entire burst minus Tibbers. His Ult somehow went from almost dead to almost full health while it was attacking the enemy. I even asked the Yorick if his ult description says it heals off of enemy champs but according to him it doesn't. So I was just wondering what was up with the never dieing ult that tanked around 10-15 tower shots. So if this isn't really a bug sorry for making a post but it just seemed kind of weird to most people in the game.