the first grand twitter -> masto migration was not last year; it was in 2017. this migration brought in a lot of gay & trans people who were boycotting twitter over moderation policies on organized harassment by gamergaters, TERFs, and the alt right, and it's a major reason why fedi is *so* trans so it feels really weird for people to act like it never happened just because they're noobs.
the first grand twitter -> fedi migration was in the identica days, back when twitter was competing with google+ and ello... so 2009ish maybe? this was related to excitement over distributed, noncommercial social media, and brought in a lot of anticapitalist & FOSS people. this was during a period when twitter still had the fail whale, to give you a sense for the time.
there has been at least one major migration-wave to the fediverse per year since 2017, more or less evenly split between people moving because they don't want to give a rich guy more money and people moving because centralized moderation sucks. the one that happened after musk bought twitter was unusually large but not different in kind from any of the others.