One things that always surprises me is how much of the movement of the stock market is structural. From this piece, Does Options Expiration Week Equal Stock Market Strength?.
For the last 21 years, the week leading up to options expiration has been consistently bullish and the weeks before and after bearish in terms of both returns and percentage positive.
Actually, that doesn’t surprise me. What does is how few people, outside of many trading professionals, know about those structural issues. Then again, how many people outside of trading professionals even know how options work?