Not only are these two defensive-minded teams, but they also rank first and second in terms of slowest pace in the Mountain West Conference. 
San Diego State is an upper tier defensive team ranking 12th in the country, giving up 60.7 points per game. Air Force ranks among the bottom teams scoring-wise at 58.9 points. The Falcons have failed to reach 60 points in four of their last six games. They rate 342nd in the nation in scoring. 
Air Force catches a break in that San Diego State won't have their leading scorer, senior Matt Mitchell. He has a knee injury and won't play for at least another week. MItchell averages 15.3 points, the only Aztec player to average more than 13 points. MItchell had played in 114 out of a possible 114 games for the Aztecs before his injury. His likely replacement is Keshad Johnson, who recently returned from a shoulder injury and averages only 3.2 points. 
There are several pertinent Under trends that dovetail together - The Under is 7-1 the last eight times San Diego State has played a sub .500 opponent and the Under is 22-7 the past 29 times Air Force has met a team with a winning record.