Because brakes create friction, and heat is a byproduct of friction.
You are decelerating, and as you do so you are transfering quite a bit of energy to the brakes. That is why brakes can get very hot. The kinetic energy of the moving car is converted to friction on the brake pads.
The brake discs or drums get hot when you apply the brakes and then the heat transfers out to the rims
because they are constantly being used
Friction reduces the speed of a moving body by opposing its motion. For example, friction between the tires of a car and the road slows down the car's acceleration. Friction between a book sliding on a table surface slows down its movement.
You went thru a deep puddle with hot brakes. The disc gets doused in cold water and warps.
The incandescence of a meteor is caused by friction between the surface of the meteoritic body and Earth's atmospheric gas. The gas does not have to be hot to cause the "burning" the energy for this comes from the speed of the meteor as friction with the gas slows it down. Rather like car/bicycle brakes heating up when they are applied.
Car brakes work by applying friction between two surfaces converting the motion (kinetic energy) to heat and slowing down the car. Traveling a downgrade for long distances can result in the brakes overheating and the friction surfaces grinding down and/or burning out. The correct thing to do is to downshift your transmission to prevent the vehicle from gaining too much downhill speed.
just keep driving, if your have not checked your brakes then i would get a professional or you could take off your tyre and look at the brakes. consider getting new ones if they are looking damaged
1 way trailer brakes can get you killed...If you are going down a long steep grade (hill) your car/truck brakes (which are only made to stop your car or truck) will get hot and glaze over and eventually completely fail if your trailer brakes don't work properly.You will probably go over the side of the hill,crash into something and kill yourself and possibly other people.If it don't work,don't tow it.
It could be air in the brake line, or the brakes getting too hot from too much friction by using the brakes on a steep hill or riding the brakes, or there could be a problem with the master cylinder.
make sure it's not the rear brakes hanging up, see if they feel hot, if you have drum brakes on the back a wheel cylinder could be hanging up.