Indiana has an underrated defense. The Hooisers can't stop high-powered offenses, but they are tough versus weak attacks holding Eastern Illinois, Connecticut, Rutgers and Northwestern to a combined six points! 
Purdue's offense has been ruined because of multiple QB injuries and an injury to Rondale Moore, who I ranked as the top wide receiver in the country. The Boilermakers are terrible on the ground ranking 128th, averaging fewer than 75 rushing yards per game. 
The Boilermakers have been respectable on defense versus middle-to-lower Big Ten foes. Discounting last week's game against Wisconsin, Purdue had given up an average of 22.6 points in its last five Big Ten games.
There are two other key factors that favor an Under. The weather forecast is calling for a 90 percent chance of rain with wind in the 13-15 mph range. That's especially bothersome to Indiana, which is a passing team. Note the teams are playing on a grass field, too, which is a plus for the Under.
Also this is a rivalry matchup so defensive intensity should be up.