What the movie makers never explained . . .

I was thinking Sarah's dad, but that means she'd have to not be born yet in 1972. That would make her 11 at most in 1984, so it doesn't exactly work. Suspend your disbelief for a moment, though. It would make an interesting twist. The company that owned Dalos was later bought out by Cyberdyne, and the "bug" that caused the robots to "kill all humans," as Bender would say, lived on in some AI chip they continued to use.