If a target that the Warden is roaring at despawns, the Warden will get stuck in of continuous roaring. Normally this wouldn't be an issue, but Creepers exploding does not count them as actually dying so the Warden's AI gets confused. 

In summary, if a Creeper explodes while the Warden is roaring at it, the Warden will get stuck in a roar loop.

Video below should show this.