My earnings were also down in October, but worse than that, I did not receive the payment in October from September's earnings.
There is a bug in their system and it is possible that there are multiple bugs. I reported the payout issue and they acknowledged it and said they are working on it.
Here is the link to their Known Issues page.
AdSense Known Issues
They claimed that they have resolved the issue with zero payouts, but I have not yet seen the October payment.
Have you checked your traffic? Your traffic may also be down due to this year's endless updates.
"It's inexcusable for scientists to torture animals; let them make their experiments on journalists and politicians." -Henrik Ibsen