Master cylinder
Brake fluid expands when getting hot. Normally, without pressure on the brake pedal, there is an open connection from the brake lines to the brake fluid reservoir through the master cylinder. If for some reason (pistons in master cylinder not fully return) this connection is blocked the fluid expansion leads to pressure on the pistons in the callipers and eventually to locking up of the brakes.
Restriction in the movement of the pistons is caused by worn seals, corrosion of the cylinder, or contamination of the fluid. In any case the master cylinder has to be overhauled or replaced. Note that there are several different models of the master cylinder.

Brake lines.
Collect al brake fluid in a clean jar, if there is any sign of contamination all flexible brake
lines have to be replaced. If not inspect the lines for small cracks and replace if you find them.

Callipers.
Since your car is not very new and you like to wrench , I would strongly recommend to inspect/overhaul the callipers. It only requires some cheap rubber seals and some time. Make sure you have the right seals for your calliper, again there are different brands. If the bores or pistons are scratched the whole calliper should be replaced. Also make sure there are no signs of overheating of the piston (discolouring).

Rotors.
If the rotors have overheated or worn thin, you should always replace them. Not to expensive and essential for your (and others) safety.

Rob