Flooded Almost!

Left the Motorhome unattended for a few hours and returned to find the shower tub filled with water! The grey tank was almost empty before I left and I'm curious to why I almost completely flooded my RV. Earlier I took a shower and shut off the water flow from my shower head shutoff valve instead of the faucet valves. I don't usually do this, I just forgot to turn it off from the main valve. There was no water flowing when I exited the shower.

What caused my grey tanks to fill and overflow? There wasn't any water coming form the shower head when I discovered that I left the valves open. I always thought the shower head shut off valve did just that, stopped the flow of water and the filling of the tanks.

I'm stumped.