UPDATE: It turns out there's nothing to this after all. Joshua Hedlund has the whole story here.
Here's the weirdest chart you're likely to come across today. It comes from a paper by Luc Laeven and Fabián Valencia and shows the starting month of all modern banking crises (i.e., those after 1970). By a huge margin, banking crises mostly start in the second half of the year, and the overwhelmingly most dangerous month is September.
Greg Ip tentatively suggests this might have something to do with the fact that October 30th marks the fiscal year-end for American mutual funds, but he's not really convinced, and I don't think I am either. Perhaps we could get a better answer if we grouped crises not by month, but by astrological sign. Are all those September crises in Virgo or Libra? Inquiring minds want to know.
UPDATE: I suppose there are lots of other things that show a similar pattern and thus might be related, but a reader sends along this one, which is sort of interesting. Maybe it's all about the oil!