Why Do Financial Crises Happen in the Fall?

It's likely a combination of factors. Here's my theory:

Real estate: people tend to time their home purchases and apartment leases to the school year. For families, it's hard to relocate in the middle of the school year, for new college grads, your housing plans were determined by school year and this habit carries over for several years.

Business: for businesses whose fiscal years follow the calendar year, money gets tight right around fall because planning and budgeting happens more or less quarterly. I worked for the Washington Post (Kaplan), and like clockwork every year for 5 years, there were budget and hiring freezes every Sept 1st.

Commerce: combine real estate and business from above and you will get a picture of what people have to spend for the most important season in retail - Christmas.