Game Mechanics: City of Mist: Abnormal Black Reaver trains

[Csihar]

I've noticed lately in City of Mist that the house that holds the Black Reavers will periodically train out (Black Reaver + surrounding mobs) without any clear indication of why. The train runs out but doesn't initially aggro moat group, so it's not them.

Today I was in a group at Temple and all of the sudden we got a Reaver train which headed right for us. It was quite clear that we had aggroed it from Temple area somehow. I think the exact trigger was 'Solon's Bewitching Bravura' being cast from the Temple ground on a mob on top of the first story of the temple. About 1-2 minutes later the uninvited guests showed up. Perhaps that's where the cause of these odd trains lies?