Even though the TV is "off", it is still on, or at least parts of it are still on. The parts that are on are for remote sensing and things like that. I'm pretty sure, the HDMI is sending and looking for an on command from a source, and the PC is doing a handshake with it.
I'm also pretty sure that in the TV's menu, there is a command to disable automatic turn on, when it senses a handshake from a source component. Look for this in the menu, turn it off, and the TV will stop doing what its doing.
Wayner