Forced because you can't move Blizzcon date once machine is running. They start selling tickets around April, first preparation likely much earlier. Not to mention first week of November became tradition at some point.
Imo they just needed something exciting for 8.2.5 and SL drought in general. Legion had Allied Races, scalled world, legendary vendor useful for Mage Tower, new battleground. BfA had massive XP boost, introducing reputation boosts to timewalking, corruption vendor (must be more, I remember they handled drought very well).
But SL already made systems alt friendly in 9.1.5 and 9.2 don't require massive catch ups, only thing they could add in 8.2.5 is maybe Renown 80 token and further Anima/offerings increase. Barely anyone cares about new arena. So it's perfect time for something like cross faction, especially if it's instanced version that doesn't require big changes.