I think it might have to do with fumbles. Here are his rushing stats over the years:
2010: 16 games, 327 carries, 1616 yards, 4.9 ypc, 16 TDs, 89 First downs, 3 fumbles, 2 fumbles lost
2011: 13 games, 278 carries, 1,224 yards, 4.4 ypc, 10 TDs, 58 First Downs, 4 Fumbles, 2 Fumbles Lost (he had 1 fumble lost on a reception too)
2012: 8 games, 192 carries, 770 yards, 4.0 ypc, 10 TDs, 42 First Downs, 0 Fumbles
The Texans have been exceptional this year with regard to turnovers, +6 so far this year. Foster had a history of fumbling the ball in college at Tennessee, and he did have a few over the past few years, but this change in style or strategy (however you want to look at it) appears to benefit the team more than those few extra yards he may get. Call me crazy if you wish, but I think this is part of Kubiak's strategy to maintain ball control throughout the game, and keep Arian's confidence up.