This is a "Wild Mass Guess" entry, where we pull out all the sanity stops on theorizing. The regular entry on this topic is elsewhere. Please see this programme note.
Vanduul attacking humans.
Janus: We know Janus was willing to do anything to save the humans, which leads me to the theory that they might have landed on a Vanduul planet. In a desperate struggle to save the humans, Janus terraformed the planet, killing the Vanduul on it. This would explain why the Vanduul don't exactly love us.
Terraforming is a long process requiring a lot of resources, not something that can be done on short notice. The Vanduul just don't seem to like anyone very much (although they do seem to have a bit more of a mad-on for Humans than the other races).