Alistar getting free ap from nowhere.

So after patch I was playing an ARAM, and at some point Alistar started to solo kill me easily while building full tank (I was Corki), then I noticed his stats, he was getting free high quantities of ap from nowhere while building full tank. I don't know if this is only happening in ARAM, or if it's a champion specific thing, but yea it wasn't fun.

Edit: so it happens at random, saw other champs getting random AP quantities aswell lol.