Waves are everywhere. The TV picks up whatever waves it can. Some of those waves are signals meant to transmit an image (eg from a broadcast tower), others are just random noise in our environment.
It's been awhile since I've messed about with this, so I don't remember (and you may not either, so this is an open question), but wouldn't it produce the effect even if disconnected from an antenna?
If so...Would the same principle be in play of it picking up on general EM waves to cause the effect?
the effect can change slightly if you unplug or touch the antenna or the TVs socket for it, because it may change what contributes to the signal noise and how much. It can for example become brighter and the pitch of the audio noise can change.
AM car radio can pick up the waves from your cars electrical alternator as well, which can cause the static to rise and fall in pitch as you speed up and slow down.
The display on the screen is the strongest signal. Without a strong signal from a TV tower, you just get noise from 60 Hz AC running through the wall, or radio towers, or power lines, or whatever else makes that radio noise.
Afaik the antenna is picking up the background waves/radiation and the TV is displaying that background waves/radiation. If you disconnect the antenna, the TV will have no signal to display, it'll be as blank as it can get.