Simply that moving the TV exposes it to a different localized magnetic field -- just like a compass inside a building. This could also be the result of moving a

Simply that moving the TV exposes it to a different localized magnetic field -- just like a compass inside a building. This could also be the result of moving around big loudspeakers near the TV (especially if they aren't magnetically shielded). Built-in degaussing circuits fire when you turn on the TV presumably because (a) it disrupts the image, so best to do it at turn-on than randomly while you are watching, and (b) because you are unlikely to move the TV while it's on.