Newtons third law states that every action has an equal and opposite reaction.
Knowing that the shield is made of the strongest material on the planet and that Captain America has super human strength, it is no stretch of the imagination that the resulting force of the throw could potentially bounce back off a HARD surface.
I've not seen a case where he bounces it off a soft surface, like a human... this to me would dampen the kinetic energy... like putting your finger in a newtons cradle and watching it all stop.
In Age of Ultron, Captain America has a much more sophisticated suit that uses magnets to return the shield to him... but in the films prior to that suit... Newtons 3rd law seems to be the answer.