It seems it's the new MO for networks, popular shows get all new shows during the sweeps months, but get a little rest between March to mid-April so networks can air repeats or air the all the leftovers from the cancelled shows, try new shows, etc.
What the other poster just said...March is not a sweeps month, most series take a break so they can have 6-10 new episodes straight from Spring until early summer. I kind of use these periods to get caught up on shows I missed.