Were you using a metal drip tip?
Some metal drip tips can go too far into the chamber when inserted and make contact with the positive post, or snipped wire on the back of the post if you bent it up.
Sometimes drip tips have some "play" to them, and it wasn't making contact until you nudged it the right way while vaping to finally make a connection and cause the short. Could be why it happened out of the blue...
I am, but it doesn't go very far into the chamber. It doesn't go below the lip of the RM2, I checked that before buying the tip.