I'm with those who suspect possibly an animal in the road. It sounds like she probably flipped over the handlebars, no? The only ways this can happen are: 1) front brake is suddenly applied hard enough to lock it, 2) a crack or pothole causes the front wheel to suddenly stop, or 3) front wheel hits some obstacle on the road.
Her concussion could allow something like a deer strike to occur but not register in her memory. I crashed a bike as a teenager and received a moderate concussion, and for days and weeks following, I had no idea how it occurred (for several hours I didn't even know where I was or what was happening). I later began to recall some details, until eventually I gained a pretty good idea of how the crash happened. Maybe your wife will eventually gain some memory of it, but maybe not.
If she simply passed out prior to the crash, that might square with what you know from the post-crash analysis. But this seems unlikely, unless she's had fainting spells or something of that nature ever before, even once.
There could be some merit to the theory of distraction from the derailler's function, but given her extensive riding experience this seems unlikely. You'd expect this more from a new rider.