RF brake locked up. Why?

It shouldda had a horrendous pull to that side. If it didn't then check the other side;they mustabin BOTH dragging. And if that is true, it points to a non-functioning Compensating port, due to the M/C piston not parking far enough to the rear.

But if in fact only the one side was dragging, then it could be for several reasons, as already mentioned. Or even a wrong-side self adjuster installed.

BTW; cooked bonded shoes may be trash; the glue may have been compromised. Linings can be de-glazed and if normal underneath,reused. If dragging a small screwdriver over them with modest pressure lifts flakes or digs a rut, then they are toast.