I may be out on a branch here, but if you've got current in the sump, chances are you have it in the main tank as well. Electricity is very fast. At any point where there's a path, it will travel. Because of that you've got a connection between your tank and sump if the water is indeed coming from the DT.

You're probably not grounded, or a path to ground, when you're in the DT. Whatever that connection is between crouching and working with the sump vice standing (perhaps on a stool?) to deal with the DT... if you've got it in one place, you've got it in the other.