The most common reason is that there is something restricting the water flow to the single handle shower faucet spout (for example, solder or something lodged in the pipe). Something is impeding the flow of water to the spout and causing it to back up and go out of the shower head. If this is the cause of shower-rise, it is possible to remove the blockage by removing the tub spout, and then feeding a plumber’s snake or a speedometer cable up through the tub drop. The act of feeding it causes it to twirl and loosen the blockage. Be sure to flush the valve before reinstalling the tub spout. Be sure to check the tub spout itself for blockage.