This is due to the force of viscosity, or better understood as air friction. It is the reason that any object will eventually stop speeding up and reach a terminal velocity. This viscosity force is often described as an expansion: F= av + bv^2 + c v^3 and so forth. Often this is approximated and the only term used is av. For a sphere falling under streamline conditions a=6*pi*viscosity*radius of the sphere.
Hope that helps.