there are loads of little bars running up and down inside the box that connect various bits with one another

it's fairly common for the boxes to become corroded and that means that the bars can come into contact with each other if there is water present thus causing all this mess

you CAN take the box out and clean it up etc, but the generally accepted idea is that you replace it for a "new" one as once it starts to play up, it's going to cause more and more grief over time