Fiesta auto gearbox problem?

My son has a Fiesta R reg 1.4 with auto gearbox I don't know much about automatics. This is the problem, the car tries to move of with a bit of force when put into D rive, it is OK when in N eutral and when put into R everse it is OK it does what I would expect an automatic to do, Can anybody explain the reason for this as I said I don't know anything about automatics. Reverse is OK, forward it wants to go, driving I believe is fine it is just the wanting to move off when stationary