One of the interesting things about sports is the unquantifiable nature of much of it. No matter how much sportswriters and analysts try to fit the entirety of a game into a box score, there are always things that can’t be measured. These things are often very important: They can comprise the “why” of winning and losing.

For instance, when North Carolina’s defense went limp in the second half of an important game against conference rivals Virginia Tech Saturday, there was no easily discernible reason behind it.

These instances, when a team’s effort flags, or when a team is much worse than the sum of its parts, are often summed up with nebulous words like “chemistry” or “momentum.” And maybe what ailed Carolina’s defense in their second-half collapse was a result of failing momentum when their team chemistry changed after QB TJ Yates left the game with a sprained ankle in the 3rd quarter. Or maybe they didn’t “believe in themselves.” Or maybe “they didn’t want it as bad as the other team.” Or maybe they were just tired.

Whatever the reason, Carolina let Tech shove the ball down their throat for the entire 4th quarter, aided by two interceptions from freshman quarterback Mike Paulus, subbing for the injured Yates. The result was a disappointing, and avoidable, 20-17 loss.

The loss was especially disappointing because the ‘Heels had the game well in hand at halftime. Even going into the fourth quarter they were up a TD, they had a total of 269 offensive yards (compared to Tech’s 180), they had two interceptions (compared to Tech’s 0), and they had held Tech’s superstar QB Tyrod Taylor (who also left the game late with an injury) to 112 yards passing and 27 yards rushing.

In fact, Carolina’s defense, along with the passing game, was the best thing about their game until the end. They looked like they had an extra step on Tech, and pitbull linebacker Mark Paschal was everywhere. Tech suffered through a lot of three-and-outs, while UNC converted big offensive plays, usually with sharp passes from Yates to star receivers Hakeem Nicks and Brandon Tate.

All of which makes the collapse so frustrating. This game could have been a proclamation, the way that UNC’s impressive defeat of Rutgers wasn’t, considering Rutgers’s slide into mediocrity. There are obvious reasons why the offense suffered after Yates left the game, but no explanations for the defensive flaccidity.