It is hard to tell exactly but it appears you are on a joint account that was overdrawn. In the terms of your account there is the "Right of/to Offset" allows them to deduct money from the positive account to "offset" the negative account. So they could have pulled money from ANY account with your name OR the other account holders name at the same bank. But they do not have to be a joint account between you two. In fact you could even have been on a joint account with another person and they could have taken money from that account.
Of all of the actions a bank can take this is perhaps the one that is the most unfair. As you are basically being penalized for something that is (hopefully) not your fault. Unlike other fees such as Overdraft or NSF fees that you as the account holder can avoid by managing your account.
You probably should be more upset with the person you were on the joint account with. This is why it is not only very important to read the terms of the account. It is also very important to know who you are getting a joint account with. If you wouldn't be willing to accept responsibility for their actions you probably shouldn't open an account with them. That goes for not only bank accounts, but loans, or other credit or anything else you may ever enter into a legal agreement with.
Now, this is figuring that this is what happened. If you are saying that they just added another person to your acccount and took your money, or transfered the money to an account that you were NOT on. Then they could be in some serious trouble.
By the way I have not seen any financial institution that did not include this "Right to Offset" in the terms of the account.