Why has September been a week time for stocks in general?

Not trying to trigger any bearish signs for this September, but I'm just curious to learn.

Most of resources available like websites and youtube videos say that stocks have been in a downtrend during september for almost a century. Investopedia says that between 1928 to 2021, S&P 500 has declined at 1% in average during september.

Obviously there have been years where september was a positive month, but is there a clear reason to why the market sees a downtrend during september?