Biting or nipping normally occurs on the rear feet and it can be common among diamondback terrapins. Biting is normally something that can start with diamondback terrapins between 3 and 4 inches. This seems to be the time that they start showing aggressive behavior. Sometime this occurs because they are still being housed in tanks for hatchlings and their isn’t enough room. Unfortunately I have seen this happen for no real reason at all. It is much more common to have this issue with females biting other females. I really haven't had an issue with males biting each other.
It is much more rare to have problems with adults nipping at each others feet. In some cases it is just one diamondback terrapin that is being picked on. You can move the turtle to another tank with another group of diamondback terrapins they may not pick on him. Other times it is just one terrapin in the tank doing the nipping and if you move her to another tank normally she will not pick on her new tank mates.
The best way to stop the nipping is to separate the biter and keep her separated for several months and then reintroduce her. Also you will need to separate the turtle(s) that have been bitten and let them heal for a couple of months. I have seen that most of the time the turtle will grow out of it but the biter must be separated to break the cycle. I believe some nipping occurs because too many diamondback terrapins are kept in a small tank and moving them into a larger tank can sometimes break the cycle once the nipped terrapins feet have healed.