After spiking up to 10 million subscribers last quarter, WoW has fallen back down to 7.1 million subscribers. As usual, everyone in the community takes this as evidence that whatever new element in Warlords of Draenor that they dislike is ruining the game. (I blame Active Mitigation personally, clearly inconsistent tanking is driving people away.)
I think the most likely explanation is that the Warlords of Draenor nostalgia trip plus the 10th Anniversary got a lot of old players to give the expansion a shot. They played through the expansion, then realized that WoW is more or less the same game as when they left. So they quit again, for pretty much the same reasons as before.
Of course, there could be other reasons. The big element that everyone is mentioning is Garrisons. I'm not a major fan of garrisons, but at the same time I don't think they're that bad. They're fairly low maintenance, and easily ignorable, in my view.
If I had to choose to an element which "caused" the drop, I would say that many new elements are combining to weaken guilds, and make the game excessively transient. Things like the Party Finder, cross-server groups, Personal Loot, and maybe even Flexible raiding. In my mind, guilds are a major element in making the game "sticky". Not being in a guild, not being in a community where you see the same people on a regular basis, makes it much easier to drop away.
Of course, we don't have access to data on who exactly is leaving. Blizzard does though, and the best way to find out what caused the drop is to see how Blizzard reacts. The classic example here is Cataclysm. Blizzard made dungeons very difficult, Ghostcrawler put out a post defending that difficulty, sub numbers came out, and two weeks later all the dungeons got severely nerfed.
So we will see what happens with Patch 6.2. Thus far, it looks like Blizzard's main focus is trying to get people to participate in multiple types of content. To me, that implies that players are "silo-ing", focusing on only one part of the game like raids, PvP, or pet battles. Players are not venturing out to try other aspects, even as they are becoming bored with their chosen activity.