I suspect it had just as much to do with the lack of pass rushers on this year's defense after Nathan Williams went down. If you are going to play man, you can only expect your secondary to maintain coverage for so long. We should really be focusing our recruiting on pass rushers. If you think back a few years, we always had great pass rushers (Gholston, Gibson, Heyward could get it done), but seemed to lack the big, floppy-bellied space eaters on the interior of the line. We've brought in good interior lineman in the previous few years (Simon and Hankins, most notably), but now it seems like we don't have any real edge rushers. Of course, I thought a guy like Melvin Fellows would be playing this role by now, but that hasn't panned out for a variety of reasons and even he is more of a hybrid lineman that can slide inside or outside. Hopefully someone like Kenny Hayes can step into that role in the future, and, as I said above, Washington is a prototypical pass rusher.
But my broader point is that when you can't get a pass rush at all, it puts your secondary in a really terrible position and exposes a lot of blemishes that you might not otherwise see if the QB were operating under duress. But there is no excuse for the horrific tackling in the secondary this season from everyone but Roby.