From what I've read, the Hessians allied with the Outcasts for weapons and assistance against the Corsairs. Not exactly sure of what happened between the factions or in-game, but at least inRP the Outcasts were trying to manipulate/replace the Hessian leadership so that they could smuggle Cardamine into Rheinland. The Hessians were enraged and cut ties, now they don't want to deal with the Outcasts and so don't let them on their bases.
