It was more that regular people started joining the internet through just paying for ISPs. Before that, most of the people just joining the internet were students, so there would be a wave of newbies at the start of the university year every September and they would get acclimatised to the culture there during the year. But once it was year-round and many more people it swamped things and the culture shifted or closed itself off.

Exactly. The "September" was an existing phenomenon, but was only limited to a couple of months every year. It became "Eternal" after the masses started finding their way to internet (well, USENET in particular).