I was in Tokyo Disney last week and was wondering the same since I suddenly saw so many You Tubers/Instagramers in Japan around the same time. I don't think there was an event or anything, but I'm guessing it's some combination of Japan tourism opening up (which happened a year ago, but most people probably couldn't plan the trip right away), weak yen (exchange rate was great for my trip), and maybe Halloween events (extra parade, special merch/food/decorations, and I was blown away by the costumes guests wore to the park -- many we thought were official characters at first glance!).
I am surprised that there are so many going now since Fantasy Springs is opening next year.