WoW senior producer John Lagrave told Eurogamer that this drop is not really a surprise, and he went on to say that they're confident that many gamers are waiting for the new expansion, Mists of Pandaria.
"We've seen a dip in subs," he said. "It certainly has to at least be attributable to The Old Republic, but it's also attributable to people who want to wait and get Mists of Pandaria, so it's not surprising."
The Mists of Pandaria expansion focuses on the new continent of Pandaria, the Pandaren, and the monk class. It will also increase the player level cap to 90. However, there is no release date for the expansion pack as of yet.