Gangplank passive proc cross map?

I was playing as Zed and I'm getting jumped by 2 people in front of dragon pit when Gangplank ults. I use my ult on one of the 2 people who are chasing me and then shadow over the wall to the exploding plant in red team's jungle. I still had some HP but all of a sudden, I just died. I was not ignited, none of them had used a damage-over-time ability and there was no AA flying at me. I checked the Death Recap and it said that the killer was Gangplank. I looked at what he used and it showed his ultimate which made sense because I was hit with one barrage but then there was also his passive. Gangplank was top lane. If I am correct, his passive can only be proc'd by his auto attacks so what happened?

I'll try to get a picture or video link up when I'm able.