A CIVPEN stands for Civil Penalty. Typically, if such a levy is placed on your bank account by the IRS then you should also receive a notice from the IRS which outlines the reason for the levy. So without further information, it would be impossible to give you an exact reason as to why this levy was placed on your account.
However, since you have indicated that you are currently set up on a payment plan with the IRS for back taxes, it is very possible that one of your payments was either missed or late, in which case the IRS does have the option of placing a levy on your account. Even though a payment plan is in place, once the terms of that payment plan are not strictly met, then a levy might be placed on your account.
If this levy is from the IRS, then you should be receiving a notice from them soon as to the reason for this levy. In the meantime, you can always contact the IRS to verify the reason for the levy, however, it sounds as though it is most likely due to a missed payment or a late payment on your current plan.
In order to have the levy released, you would once again have to work out arrangements which are acceptable to the IRS on the repayment of your past debt.
Hope this was helpful.