Probably a short, or the resistance was WAY too low. The best guess i can make without any photos of the build/box is that part of your coil was probably touching some part of the deck. Always look carefully at your coils to make sure they aren't touching the deck or the posts, and make sure that the leads going through the posts are clipped so the don't touch the coil on the other side. Depending on what batteries you were using, it may be easier to short a box mod than you might think. One of the reasons you see everyone here preaching battery safety haha, because disasters are real!