I would speculate that this is an effect of perception. DKs felt / were perceived as weak for quite a time during the TOC era, which translates into people moving away from playing their DKs and into something else.
I think perception is huge in these matters, much greater than most players give it credit. WoW players generally really, really hate to be wrong. What I mean is, they don't want to think that they made a bad choice in their character class, their gearing or their talent choice. In WoW it's somehow less embarrassing to fail at something than it is to be caught being wrong.
"You wiped on heroic Saurfang? Yeah he can be hard. You had a paladin healing the raid? What, are you crazy? I don't care if it worked for you. It's just wrong. Ugh, and look at your glyphs." As a result, rather than basing decisions on noticing empirically whether something does or does not make a difference, many players would much rather someone tell them the right or wrong way to do something.
So we see the level 40 mage that I saw in Dalaran last night, spamming trade chat asking whether Fire or Arcane does more damage. (Answer: Dude, it doesn't matter if you're level 40 and probably doesn't matter if you're level 85 unless you're going into a really tough Arena match or talking about heroic raid bosses.)
I think players switch tanks sometimes when they think the forums are telling them that they're using the weakest main tank. "Well, I want to plan for success, so clearly I need a different tank," even when in reality that choice probably affects their chance for success very little. Switching tanks might give them a 3% greater chance of success. Not standing in fires might give them a 30% greater chance of success.
Likewise, I think players sometimes drift off to an alt when their class or spec gets nerfed, just because it sucks to get nerfed and they feel somehow less potent knowing that they were nerfed, even if the effect was imperceptible for them.
Perception.The only logical reason then to bring about tank representation balance (assuming that this should be the goal of Blizzard) is to disproportionately make DK tanking attractive over Warriors and Paladins.
It's not a goal for us. There are a lot of things that can affect representation. Balance is definitely one, but as I mentioned above so is perception, and then you have tradition, play style (what's fun to play), utility when not tanking (including healing, but also dps and even PvP) and even art. It's complicated.
Now if you see a sudden shift in populations, then there might be something happening that you need to pay attention to. Something happened to upset the inertia and drive player behaviors in a certain way. It could be a lot of things. It could be perception. But it could be an actual survival of the fittest deal where raids using one class struggle and those using another class succeed, so gradually you see the latter increase over time. It's hard to separate out all of the various causes for behavior like that, but it's probably good to take notice when they do.