Early in the season, we seemed to be getting a lot more 3 and outs. When we do, the defense rests. Yes, the offense has been inconsistent which doesn't help, but many times, I see 10 yard cushions on 3rd and 5's and they take the opponent takes the first down. The drive continues, and we get tired.
Again, IIRC, we were much more aggressive in the secondary earlier in the season. Perhaps injuries to our secondary (King and Brown) factor in.
Nevertheless, I think we are vastly improved on defense over last year and especially over 2 years ago.