To activate Archavon the Stone Watcher anniversary world boss version you need to defeat the Val’kyr Hunters and stand on their ‘plates’, which light up when someone isn’t standing on them. However, it seems that if mixed faction players are standing on the four plates the lights will go out but the boss’s shield won’t drop to activate him.
Even if four players from the same faction do stand on the plates it won’t activate the boss if a player from a different faction stood on the plate first and is still standing on it. That player, me in this case, has to leave the plate and let someone from the majority faction move onto it.
I do not know if being in a group with the mixed-faction players changes this behavior as I was ungrouped when I observed this problem.