Every faction will get their turn. Although if this event is most likely meant to build up the playerbases for the xenos factions, the dev lovin' should have came before unleashing the hordes. As it stands, making the buggies and least polished faction to be the first one to get the event will send a truly terrifying message to anyone trying the game out.
Wait for it, it's going to be rough for LSM when they've their campaign and everyone else tries to play LSM too.