Some kind of Luden's Echo bug?

So I'm the Annie in this clip, and my Luden's seemed to proc on Nunu when it doesn't seem like it should've. I only used my Q and E in this clip, and neither of them were in a scenario where they could've procced the passive.

My Q hit him way too early for it to do it. Nothing auto attacked me for my E to do it. Arcane Comet (or any rune for that matter) doesn't proc spell effects. Not that it even hit him. My W and R weren't even used.

Am I missing something here, or did I encounter some weird bug? My items at the time were