SHIELDING. By tapping the piezo device you are sending a voltage pulse down the wire to the Arduino. This pulse will induce a pulse in ANY parallel wires. A shield is a metal tube or mesh around the individual wires that is grounded at 1 end and shields the signal wire. You also may be seing some effect on the ground wire as it will also change in voltage depending on size and length. Run a seperate ground from each knock sensor back to ground and put it inside the same shield as the signal wire going to the analog input.