I'm thinking it's a delayed primer strike...a mechanical malfunction as mentioned.... not a light primer strike followed by delayed ignition. If such were possible, I'm pretty sure I would have witnessed it once over the last 20 years watching hundreds of target shooters immediately open their guns after a misfire. Can't say I've ever seen anyone wait even 10 seconds before doing so.
If I had that gun in hand, and it went "click" on a live shell, I'd extract it immediately and bet there was no mark on the primer.
