It's a reference to when AOL started. Every previous year, new students got online from their university around September. Some behaved badly, but the numbers were small enough that they could be educated on 'netiquette'.

When AOL started, suddenly there were too many newcomers, and the influx of rudeness never stopped.

I guess also students are more receptive to learning new rules than the average AOL user.

https://en.m.wikipedia.org/wiki/Eternal_September

There used to be an influx of young students to the internet every September because most people used it at university for the first time. That changed when home internet became more widespread leading to the eternal September.

I think it's just a reference to the old Usenet days in the 90s when it became popular and a flood of new users came in and "ruined" it