Zilean E self-casting upon using passive to level up a team mate

I have been playing alot of Zilean recently. Two times, I can recall my E being used on myself without my selecting myself with the ability or pressing e. I believe both times this happened I was using my passive on a team mate to level them up. I suspect that there is some sort of bug in which, under cetain circumstances, when I use my passive on an ally it simultaneously self-casts E.