Update: I had the brake lever pistons lock up twice on my long ride yesterday. (Brakes were working normally at the start of the ride.) The first time was about 4 miles after the start of the ride, after a short downhill where the brakes were lightly used to control my speed. I was in a pack and as we came up to a stop sign, I pulled on the brakes to slow with the pack, and both brakes levers were locked up. I veered out of the pack, while pulling harder on the levers, and I felt a slight "pop" and they were working again.
 The second time was at the top of a 6 mile climb where I hadn't used the brakes in quite some time, I pulled on the brakes at the top of the climb, and only the right lever was stuck. (The one I purged with new mineral oil ) The Left/front brake worked normally.

It is becoming clear to me that there it is something in the brake lever hydraulic/piston side of things that becomes stuck. As soon as I can allocate the time, I will have to take one apart and see if I can find what is causing this problem. On the flip side. The ride had over 12K of climbing/descending, and on all of the long high speed descents, the brakes worked really well without any problem.

You might want to do an easy shakedown ride in the park or something before your next ride to see if it seizes up.

I hope you manage to find the source of the problem but you need to ride safely.

Its not just matter of your own personal safety but also safety to others - cant emphasize this enough!

I am interested to know as well.

Interested in a short-travel (100-130mm) FS XC/downcountry frame.