Historically, the month of September is known for poor stock performance.
Since 1950 the Dow Jones Industrial Average has seen a decline during the month of September.
Coincidence?
Probably not. There are many theories as to why the decline takes place each year. One theory is that summer months are typically light trading months because of vacations, and once fall arrives, investors are back at work and attentive to any underperforming stocks. Now that they're back they begin to unload any poorly performing positions.
Read more about it, and see other theories on this phenomenon here.