My guess is Blizzard is realizing WoW is slowly on its way out. The graphics are pretty dated, most of what they're throwing in there is nothing new, and people are running out of things to do in there. Still, on the other hand, free to level 20 is just a new way of promoting their free trial. Even from the beginning, Blizzard was handing out free seven day trial cards like there was no tomorrow, and it was easy to get to level 20 in those seven days anyway.
Truthfully, Blizzard really needs an all new franchise besides Warcraft, Starcraft, and Diablo. While those three have always been highly respected, I think something completely new would be really appreciated.