How does a screw backing itself out cause a short? Any time this has happened to me, it just translates into an anemic vape since the coil leg is making little or no contact with the post.
If I understand correctly: the coil had legs that were a little too long and when the screw backed out the coil wriggled towards the cap/cover and made contact with the cap AND center post simultaneously. It's never happened to me but I could see how it could happen. Trim your coil legs and smooth out the bottom of your screws or just switch screws to Allen headless screws with the nice conical tip. $4 for a lifetime supply of 2.5mm x 4mm long headless, stainless on ebay with free shipping no less.