Quote from kotika:
The SET is the price you would get if you make a market-on-open order in each S&P500 stock. It should be close to what the futures are doing around that time. Take a look at the december futures, they spiked to around 1290, but mostly spent the first few minutes after the open at 1280. So, there you go...
If you want a cause-and-effect explanation, this all happened because there was an opening imbalance in many stocks, with huge market orders to buy flooding the place - at that point it must have been clear that most stocks would make their high of the day at the open.
By the way, is there any way to obtain the official SET value soon after the open? I mean short of getting a Bloomie?