Because opposing offenses could pick up chunks of yards by running forward, they avoided throwing more than they absolutely had to. As a result, even though Indiana allowed 63% completions and 12.5 yards per completion, they "only" allowed 231 passing yards per game (versus 232 yards per game on the ground). Despite a decent sack rate, they only picked off 7 passes all season (less than 2% of pass attempts).
Despite average third down defense and above average sack and TFL rates, Indiana allowed 2.5 points per possession, a touchdown on a third of possessions, and still gave their offense terrible field position (inside the 28 on average).
I have an untested theory that turnovers are contagious. Obviously weather and other factors play a factor, but I also believe that when a team sees its opponent playing fast and loose with the football, they are more likely to do the same. Indiana seems to agree. Over the course of 12 games, only nine times did a team fumble away the ball; Indiana lost only three fumbles all season. One reason for this, in Indiana's case, is that they were among the nation's 25 best at protecting the quarterback - sacks and fumbles often go hand in hand. Indiana quarterbacks were sacked on only 3% of pass attempts (remember Auburn had a sack rate over 15%).
By avoiding sacks and fumbles, Indiana was able to convert a slightly below average yards per play into an average EPA per play. As for the defense, the good news is that this guy's not around anymore:
The Statistical Review breaks down teams along a number of performance categories, everything from red zone scoring to field goal percentage, and compares that performance against the rest of the FBS. All 124 teams will be reviewed from 124 to 1 by the hybrid rankings. You can find short descriptions of the stats used in the table below.