Left 4 Dead 2 Witch Horde

I'm not really sure whether this was a bug or if someone was modding the server since the source engine is really weird, but either way I did not cause this and found it in an online match. I was playing on an online server in survival mode, and for a few rounds it was normal. However afterwards it teleported everyone onto the campaign version. We were confused and so we explored a bit and there were no spawnables other than a lone molotov pickup, no physics on anything and no zombies save for the witch we saw at the end. Upon alerting it all hell breaks loose and it spawns dozens of them.