How to replicate:

Step 1 - Put a repeating command block that does /effect or /kill in a certain area (e.g. /kill @a[distance=..10])

Step 2 - Player A attacks Player C; Player B knocks back player C into the deadly area

Step 3 - Death message: "Player C didn't want to live in the same world as Player A" even though Player B hit later than Player A did.

As seen in the attachment, IluZhon knocked back MoonCapt, Ezxera, and RLYellowbasher into a /effect instant damage area, but did not credit IluZhon for the kills, and instead someone else.