Originally Posted by Selrahc
Khorne and Tzeentch aren't actually dire rivals. Tzeentch hates Nurgle a lot more...
It's Tzeentch's fault because it's Tzeentch's fault.
This is a result of one of his schemes to get Khorne and a Culture (unwitting) proxy to fight to assess the strength of the Culture. Either the Culture wins and the Khorne fleet gets wiped or the Culture loses a pawn and gets angry at Khorne. And Tzeentch gets data on their combat capability in either case.
There's probably a Tzeentch ship with a couple of Daemons sitting in the warp nearby to see if the Khorne guys get back.
Sure, his followers probably lost some ships when fighting that fleet to make them run into the arms of the Culture, but that increases his military assessment's accuracy (since his minions would know how capable the Khorne fleet is after fighting them).
It's almost all win for him. Apart from losing a few followers, but eh, followers.
And best of all, it's certainly holy-***-complicated enough that only Tzeentch might contemplate something that crazy and actually manage to pull it off.
So, tl; dr is, it's Tzeentch's fault because its more hilarious that way.