Think I'll say none of those reasons, and I'm not going to suggest people aren't doing all content either. But I will say that content drought is a subjective thing, to me it never really occurs even though I no longer raid nor pvp. LFR isn't my scene, and I only liked pvping when I was with by friends and moved straight to an rp realm the moment they left wow. I find my content in exploring, doing quests even if they aren't going to rewards me any upgrades, rping when I don't feel I'm intruding on other people's rp scenarios, and other stuff nobody would really consider 'content'. So to me I've never really had a point I go "I'm out of things to do.", I get bored of course and go away for a while to play other games then come back to play when the mood hits me.
I do understand to those that only find raiding their thing or other aspects that the time between expansions is rather irksome. I feel a sense of wanting the next expansion to turn up sooner, not cause I feel a need for new content but cause I see what's on the horizon and want to be there trying it out for myself. Which I find is the impulse that triggers the belief the content we have is no longer enough, even if in the case of those who come to WoW for X will find there is a lack of relevant X while they wait for the next big expansion.
Blizz haven't done well to keep the schedule they had hoped to of an expansion a year (or was it two?), so I can understand why people might find it something to lambaste them for. But I find I rather they take their time and deliver great content than rush if it gives us another WoD, not that I'm totally adverse to this expansion as it did have things I enjoy doing. Too much solo garrison and orc killing (I do like variety in my homicidal rampages for loot) being my only gripes, but then I could point out flaws in every game ever.
So I think the question about content droughts is, first, what content is it you're speaking of? Raiding, PvP, basically Endgame from what I've seen. Which leads to what causes such content to be consumed at a rate that the current release of expansions can't keep up? I've always noticed the community has gotten better and better at bull rushing content that in the past they might have taken longer at, but that's subjective and objectivity would be required to find a root cause. Then of course the solution, what would actually help solve the drought? I don't think expansions should be rushed, but I find that a simpler solution would be to have content (quests, invasions, raid/scenario) of the pre patch added ahead of the pre patch what adds the new talents and stuff. It might not work well due to it tending to be balanced around the talents and stuff that comes with it normally but it would give some people a bit more of what they want to break up the months between last relevant tier and the expansion. But I find it's something they could think about if they felt the wait time was getting intolerable to the community.