Just to chime in weeks after the event...
I had an issue with the rear brake on my bike a few years ago. Going along at national speed it stuck on but the bike just felt suspiciously sluggish, then when I came to a complete stop the restart was obviously even more suspicious, setting off with the handbrake on always does feel odd. It stank. The brake had seen a good thrashing and I assumed the rubber seals in the caliper had a right good baking. I stopped to let it cool down and see if I could free it off, but just by waiting it freed off. The seals didn't actually completely die and leak but I replaced them anyway. It did it again.
In the end I think it was due to a spec of some rotten detritus in the master cylinder. There are two little holes which I presume allows brake fluid to flow in and out of the system from the reservoir, one of which had this bit of a blockage - the hole was tiny anyway. I think maybe if the return hole was blocked then hot expanding brake fluid wasn't allowed to go anywhere except to push the pistons out and the brake on. Does this sound about right anyone?
Anyway. Could it be something like this? On my bike it was something that happened only on occasion, I think it happened one other time previously as well but sorted itself out no drama (so maybe 3 times total), but after removing that spec of dust it hasn't happened since.