The running back landscape changed tremendously in a matter of weeks. Before the bowl games this was a terribly weak draft class for running backs featuring good players, but nothing spectacular (Mike Hart, Matt Forte, Chris Johnson, etc).
Now though you've got serious talent available: McFadden, Jones, Mendenhall, Rice, Slaton, Charles, K. Smith, Stewart, and J. Davis have all declared or are reportedly declaring. I can't help but imagine part of the reason for some many 1st to 3rd round guys declaring is the weakness in this year's senior class and the higher grades/projects they subsequently received from the NFL's advisory committee.
Next year's class may end up being the weak one it appears.