Because a lienholder has an insurable interest in a vehicle, they also have minimum coverage limits that must be maintained on the car. The minimum coverage requirements should be stated in your loan documents. In addition, the loan officer or finance professional at the bank should have gone over the coverage requirements at the time that the loan was originated.
If they discover that a vehicle has no coverage, or that the coverage does not meet their minimum limits, they will purchase coverage to ensure that their interest in the vehicle is protected. This type of coverage is generally more expensive than the coverage you would acquire on your own. It appears that the cost of this coverage is now being passed on to you.