Ahh, you've met one of those so called "spamliners"
They simply exploit an unfixed bug with connecting pax.
As long as I know, they create their own alliances and open hubs at every airport they fly to, multiplying the game-generated blue pax. I know more or less how it works, but I don't know how they build up their airline in order not to lose money at the beginning
They may be a bit annoying if you value realism but most of the time they don't pose an actual threat to your airline because they mostly make their profits out of their own blue passengers