What event or sequence of events caused the simultaneous awarding of the generalist badge to 75 users?
There is a requirement that a site must fulfil for Generalist badges to be awarded, namely that the top-40 tags have 201 questions or more (source). Academia passed this threshold yesterday¹, and hence everyone eligible for this badge² received it at the same time.
¹ with this question (there are two tags with 201 questions, and this is the most recent one in those tags)
² which were quite a lot by the way; only a handful of sites on the network have more generalists.