That is namely hat I was getting at for the trigger.
Mostly because, quite simply put, it would be much easier for horde players to take part if the Alliance moved first. The Saur-Sylv line ould be erased since the Alliance is the aggressor and both parties can concur the blue bois need to be punished.
As for how it goes, it could actually be something simple and small escalating in dramatic fashion. Say, a farmer runs up to Huojin he remebers and goes “The Alliance returned with an army and is trying to dictate things we did not agree with!” and said Huojin, in turn, comes back with proper Horde backing with the intent to evict what they, and most pandaren present, would at that moment see as an Imperialistic Alliance presence.