dripping shower

What causes a shower head to spontaneously start dripping? The shower's water is totally turned off at the single lever control at the shower and at the control for the shower wand which is located approximately 6 feet away. It happens when no one has used the shower for hours or even a day or...

Old house, old bathroom, 50 years old. Was installing new tile and had to move shower from one wall to another. My brother helped me. He moved from copper to flex lines. Installed it with shark bites. See pics. Now: it all works fine. BUT... Turn on the shower only - shower works, faucet is OFF...