Water comes out of shower when turned off

I rebuilt our shower and installed a new Moen shower-only faucet. With the actual faucet handle in the out of the box 'off' position (pointed at 6:00), I turned on the hot water line in the floor. No problem. When I turned on the cold water line in the floor, water began coming out of the shower head even with the handle still off.



Any ideas what could cause this and how to fix it?



Thanks

-Steve

Water comes out of shower when turned off