It's Friday late evening, and I am trading options. The market closes in a few hours, and I am monitoring my positions before the options expiration. I am watching SPY trade at $601.50, and I notice something strange: massive open interest sits at the $600 strike. Over the next hour, despite no obvious news, SPY drifts down to close at $600.08. On Monday, I see the same pattern repeat — the stock closed within pennies of a high open interest strike. Is this a coincidence or is there an explaination about this behaviour?