Why were the screws set in pairs in my drywall?
Like others have said in comments, drywall was commonly nailed in pairs to help prevent punch-through--hammers had a tendency to crush the drywall and weaken the hold of the nail. When screws were first adopted the practice was continued until it was known that precisely set flute-head screws hold better than nail heads, or until old-timers retired away.
Fun fact: Purpose-built drywall hammers had convex (mildly domed) head faces to help set nail heads below flush without tearing the drywall surface, so that tapers could cover them with compound.