Before the ubiquity of the internet, in-person marketing and promotion was a huge tool for yoyo companies. Company-sponsored teams used to be more visible as promoters and demonstrators than as contest teams. That goes all the way back to Donald Duncan. Even before Duncan, Pedro Flores used his talent as a teacher/demonstrator to sell yoyos. Duncan expanded that and hired a team of dedicated demonstrators (including Flores) to travel around and create interest in yoyoing. Those promoters played a big role in pretty much every yoyo boom up through the 90s boom.
In the 90s boom, teams like World Team Proyo and THP (Team High Performance) did a lot of that stuff, which they mention in the AYYA documentary. A lot of current figures in the yoyo world started in those roles. For example, Hans and Ben from YYF and John Higby were on the Proyo Team, and Yuuki Spencer and Paul Han were on THP. There were also local demonstrators who didn’t travel around for the big companies, or independent demonstrators doing school programs or things like that. For example, Doctor Popular started off doing demos for a mall kiosk or something like that. You can still see the performance-oriented skills these players picked up then, especially with players who focus more on shows than competitions (i.e. Ben, Higby, Steve Brown, etc).
You can hear one of the businessmen heavily involved in the boom talking a little about it in this video. That is mostly a promo/recruiting talk for whatever company he is with now (or was at that time), but at the beginning he talks a little about how he marketed yoyos during the boom. He is the guy who ran THP, which he doesn’t really talk about in the video, but you can see a picture of them in one of his slides and get an idea of how they were involved in running the promotions he is talking about. He does talk about the importance of putting a local teaching structure in place to keep kids interested.
To some extent, videos also played a role, just on VHS or DVD instead of YouTube. I remember when the boom hit our school, our teacher brought in a video for us to watch in class teaching the basics up to stuff like Split the Atom and Rock the Cradle in the Eiffel Tower.
As for why the popularity exploded at that particular time, who knows. It was one of those toy booms that seem to start and end unpredictably, like with a lot of toys (Tamagotchi, Tickle-Me-Elmo, Beanie Babies, etc). The suddenness and unpredictability of it was part of what put some companies out of business when it ended. In-person networks like local clubs and traveling demonstrators facilitated a lot of the connections and got the word out the way the internet does now, though. Those things also still exist alongside the online network and still help bring a lot of new players into the online network.