1 in about 2millon for this to happen, the DOT 5 fluid is not as harsh as the 4, if the rear is sticking it's time to replace the fluid on the front too stop this from happning there.

 

Brake fluid real old? Caliper pistons sticking? Moisture in the fluid will expand when it gets hot causing the pads topush harder onthe rotor creating more heat and more expansion.Sticking pistons can also cause the problem.From experience, when the caliper getsreal hot the seals will fail, brake fluid will get on the very hot rotor and the fluid does burn. Yup, nice little fire going on the side of the road in the middle of nowhere. Looked real funny removing shirtand using it to beat out the flames, but it worked. Yeah, the shirt was ruined.

 

I agree with a rebuild, however another cause could be the brakes just need to be bled. Air in the lines and trapped in the caliper itself, will expand with heat. In the best condition, the pads will be lightly touching the rotor, a little heat can start the reaction and cause the pads to be in harder contact til they lock up. Assure the piston is fully released, if not the return orifice can be blocked holding the brake on all the time. Good Luck!

 

Hearing that the bike has been sitting for so long, I'm going to agree with Biker Trash and cHarley (Why would you ever disagree with those two anyway?).

I'm betting the piston in that caliper is sticking and needs to be replaced/rebuilt. If you don't do so soon, the rotor will crack. Usually the cracks form between the cross-drilled holes in the rotor. Look it over real close and replace if nessasary.

 

A locked up rear wheel is one of the ugliest ways to die. Get it fixed.