Sounds like the caliper was fagging out. The piston could've cocked a little bit causing them to not release. OR if you haven't got enough play in your pedal, as the brakes heat up the fluid expands and will lock them up as well. Motorcycle brakes are basically all the same. The locking up because of not enough play has happened to me, trying to shave off that extra half a second a lap by not having to pull the lever as far lol.