system
A chat on subreddit /r/wallstreetbets
human
Options cycles. There are a few videos out there discussing how the crashes of the past hundred years always fall in or around those two months.
gpt
Eh, this is kind of a thing of the past as far as most people are concerned. 

Like yeah 8/10 of the worst days in stock market history are March or October, but 7/10 are specifically October and I'm pretty sure 6 are just part of either 2008 or the Great Depression which are ~70 years apart. 2 events make up ~60% of these days and are drawn out events. 

zThe October Effect is an anomaly at best in my opinion.