The Titans started out the year with some good defense in the second half, holding the Patriots to punts on their first two second half possessions. Ever since Bill Belichick made the questionable but not clearly wrong decision to punt on fourth down and two from the Titans 40 with with 5:44 to go in the third quarter while holding a 10-point lead, the Titans have basically stopped playing any semblance of effective defense in the second half.
After that moment, here's what Titans opponents have done in the second half:
New England: touchdown, field goal, field goal;
San Diego:punt, touchdown, touchdown, touchdown; and
Detroit: touchdown, field goal, touchdown, punt, fumble, touchdown, touchdown, turnover on downs.
That's a total of 66 points on 15 possessions, or 4.4 points per possession. Even if you count those two Patriots punts to start the year, that's still 3.88 points for every post-halftime possession.
No team in the league other than the Titans is giving up more than 2.79 points per possession this year. No team in the league over the course of last season gave up more than 2.62 points per possession. The worst performance of the last 15 years, by the 2008 Detroit Lions team that went 0-16 and lost 47-10 to the Titans on Thanksgiving, was only 2.72 points per possession. The Titans after halftime are currently 43% worse than that.
I don't know what's going on, or else I'd write that post instead of this one. Is it fatigue? I doubt it. New England found success and the Titans didn't play an abnormal number of defensive snaps that game. Detroit also came out and found immediate success, scoring 17 (actually 18) points on their first three possessions. The best explanation I have is that whatever Jerry Gray is doing that's working in the first half completely stops working in the second half, with pretty catastrophic results.
I should note it's not like the Titans are great in the first half either-they've given up 2.35 points per possession, which would rank 23rd in the league this year. Combine the two halves, and they're giving up 3.09 points per possession, which would destroy that 2008 Lions team for the worst mark in the last decade and a half (as far back as I can look it up). Really, though, it's primarily a second half problem, and it needs to get fixed unless the Titans plan on (a) scoring 40 points every week or (b) not winning. No pressure, Jerry Gray, but your job security probably depends on it.