Ps3 To LCD Monitor With HDMI TO DVI Adapter Weird Problem?

So i got the HDMI to DVI Adapter for my PC monitor and it will only show my PS3 video feed if i have the RCA Cable Video plugged into my LCD TV if i unplug the RCA Video Cable from the TV the PC Monitor will lose the video from the Adapter on the PC Monitor why is this?

You have to switch the video settings to HDMI