Most likely cause would be the box bottom touching something (maybe the lug from one of the sockets) and shorting it out. So, I’d start by looking at anything that might come into contact with the bottom as it gets screwed down. If it’s a part that can’t be repositioned easily, try putting two layers of electricians tape on the bottom around where the contact would be.
Welcome to the forum. It’s generally a good idea to post some “gut shots” when asking for help troubleshooting.