why do my pedals keep turning when i freewheel?
The friction between the bearings, pawls and lubricant in a freehub or freewheel are usually enough to turn the pedals when the wheel is spinning. If you hold onto the pedal and the wheel freewheels normally, probably there isn't anything to worry about. 

Of course, if the wheel stops freewheeling altogether or the friction to resist the wheels turning becomes noticeable, then inspect the freehub bearings and pawls. Increased resistance at the freehub/freewheel will often result in the rear derailleur being pulled completely taut when the pedals are stopped.