It is also common in a number of the other MVP men. Personally I think it is a combination of focusing on full extension and long powerful strides at the start as well as a natural pattern brought about by the copious volume of sled and hills where you almost have to run with low heel recovery.
I read an interview where Bolt states that Mills was trying to get Bolt NOT to drag his foot, saying it could slow him down. Minor I would think considering how beneficial full extension is. Bolt was also coached to recover the rear foot fast once block clearance has occurred. That is straight from Bud Winter as well interestingly.