This is true, the economic market has been undergoing a crash down since August. This has been caused by various factors such as the big trade deficit in the USA. It may also be attributed to computer trading which makes the market more volatile. Another contributing factor could be the deficit in the budget.
Sometimes the stock market has ups and sometimes it has downs. I don't think it's too much of a stretch to believe that it has reached it's down point in September. It will reach it's up point again as well if we give it time. The stock market is resilient.
September stock slump is just a trendy term to make tweets more attractive and financial news sites visited more often. It`s true that recent events turned negative on the market indexes but there are more than several reasons for that. To name a few: huge investment fund as PIMCO is investigated by the regulator, overreaction to Alibaba - dubbed the hugest-ever IPO, recent Federal Reserve announcements, ongoing political tensions in the Middle East and etc.
Journalists and commentators find it easy to follow up on previous news articles. They look again for a September slump since they reported one last year. They could just as easily be looking for a February slump or a December slump. Stocks go up and down all the time due to changes in the market. We should be wary of predicting future trends based on past experience.