Margin is calculated with respect to the price level + volatility. Last time the margin was set when ES was trading at 1000 level. That's all. Nothing special. =)
You're right. However, my experience is that a change in the 'perception of risk' (even if it is calculated by a set formula) often foreshadows a change in market behavior. Just a thing I learned in the few years of trading. The market just has this uncanny ability to surprise people with certain expectations. ;-)