and actually, I wouldn't be surprised at this point if options lead them all.....

LoL, this would be just too much for my brain

It's gotta be automated somehow. No one can keep track of the overnight underlying prices, dividend payouts, options, etc. and put it all together into what the opening price should be.
But, time after time, the indexes gap too close to the contract price (minus open interest) to be random.
They've got to be pegging it with algos or some sort of automated monitoring software.

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If I may interject...

While the underlying stocks on the S&P 500 may trade overnight, the actual index only tracks them during US market hours. Whereas the futures that trade overnight can track the individual stocks and project it's value in real time.
When the market opens, the SPX then gaps to what the value of the underlying stocks require it to be.
So... (if I understand all this correctly), the underlying stocks ultimately lead both the value of the actual index as well as the futures that correspond to them, and the gap is due to the index only tracking during US market hours.

Regards,
BlackSwan