Hi there,
Yes September is typically a weaker month than most. I don't think there is any official reason for this, but in my opinion it is because it follows the summer which is a seasonally low volume period for stocks. Many investors take a break from investing and volume drops considerably. So what you are likely seeing is more selling pressure as investors/funds, etc start rebalancing everything after a summer of lower activity.
There is also another factor - several mutual funds have September as a year end and those are likely exiting their losing positions and harvesting tax losses.
Mat