The battle was legitimate, in the end, and since there is no system for calling dibs on attacking something before someone else does, no one has broken any rules here.
Let it be known: Attacking your own faction's fief or a faction allied to your factions fief to prevent an enemy from attacking it is against the rules. This can apply to any fief that may reasonably be contested within the next 24 hours, particularly those that have already been attacked by enemies.. If you drop faction to get around this you can expect to be dealt with the same way.
Only attack a fief if you intend to fight against it. There isn't much of a reason for people to 'attack transfer' anymore, as the fief transfer system is fully functional (albeit limiting - which is possibly an intended mechanic).