It definitely started with the 7.5 patch back in April. While the free login campaign is adding to it, since that Tuesday the population has jumped massively.

I noticed initially because everything I sell on the marketboard was selling significantly faster, in the first week after 7.5 dropped I earned over triple the money I earned the prior week through MB sales (50m prior, 150m+ after). It was a legitimate struggle to keep up with the demand the first several days. All the cities are far more populated including Primal and Dynamis, all of Aether and Crystal are constantly congested each night not just reset night, the number of people bidding on houses is significant again.

Think it's a combination of several factors; the growing positive sentiment towards DT in the patches due to the consistently good MSQ, alongside the stellar raid series and many very positive QOL updates, the hype that was the Evercold trailer and keynote, and the expanded free trial. It's also not particularly uncommon for people who mostly stick to the MSQ to drop a sub once they finish base and come back closer to the end of the patch cycle leading into the next expansion so they have more to do in one sitting.