It wasn't a mere stress fracture, but a clean break near the base lobe of the 5th metatarsal, mid right foot. It's called an 'avulsion fracture' of the 5th metataral. (It is not the more serious "Jones fracture").
My mother had this same fracture, nick-named "dancer's fracture".
Why did it happen?
1) My running mechanics put huge stress on the outside of my foot. I contact the ground on the outside of the foot - directly on the 5th metatarsal.
2) It was very cold when I was running.
3) I train on the track exclusively in spikes. They have no padding, and just one thin pair of socks.
4) There likely was already a small stress fracture I wasn't aware of. I had been aware for a long time of 'sore feet' ... and there already was some damage most likely.
UPDATE: I went back an examined my spikes. I may have stepped on something. Near the point of injury I found a wad of what looked like hard clay in my spike. It was a big wad of chewing gum which had turned hard as stone on the track. That may have done it.
It may not have happened if I weren't training in spikes. Or possibly it might have because of the force of the peroneal tendon pulling on the metatarsal. I don't know.
I love training in spikes, especially with water on the track. It can run faster, keep better form and have more fun. I won't be able to do that much anymore.
All that work since October, in the shitter.
I can't open my xray CD on my Mac, but here is an xray similar injury. Should be 4-6 weeks before I can think about running.