This is because your settings are configured to have your TV as the master display. You need to (once the TV is plugged in) right click on the desktop->screen resolution
You'll see the two displays pictured. Click on the one that's not lit up, and fill in the box that says "extend the desktop on to this display." Then go to the "Multiple displays:" option and choose either extend these displays or duplicate these displays. Those options are pretty straightforward.