Mechanics is not a physics term in the first place, but a logic term. It means "natural link of cause and consequence". When something happens, it has consequences, and the sum of both a cause and its consequence is known as a mechanical phenomenon. So, the knowledge or study of the intimate behavior of something is known as the "mechanics" of said something.
Quantum Mechanics is the branch of Physics that studies the intimate behavior of the particles and phenomenons of quantic scale.