Syndra ult after death [GAMEPLAY]

I was playing Veigar and I killed enemy Syndra under my turret, a second after her death I see my health disappear and I'm dead. I see that most dmg was done by Syndras ultimate and I'm sure she didn't throw it. While we were dead Syndra asked me "How did I use my R" confirming my suspicion about it being a bug. I have no idea how to recreate it...