It would interpret a shutdown as the destruction of NORAD.
5 seconds sound clip from the WarGames (1983) movie soundboard.
You can hear this line at 01:41:02 in the Blu-ray version of the movie.
Quote context
[...]
- General, the machine has locked us out. It's sending random numbers to the silos.
- It's trying to hit the codes to launch the missiles.
- Just unplug the goddamn thing. Jesus Christ!
- It would interpret a shutdown as the destruction of NORAD.
- Computers in the silos would carry out their last instructions. They'd launch.
- Can't we disarm the missiles?
- Over a thousand of them? There's no time.
- At this rate, it'll hit the launch codes in 5.3 minutes.
- After very careful consideration, I've come to the conclusion that your new defense system sucks.
[...]