Much of the reduction might be for non-obvious reasons, like whatever happened around 1980.

Exactly. Much of the reduction is for the same reasons you no longer hear much about cults, or about hijacking planes to Cuba (or about anarchists car-bombing Wall Street or trying to shoot the Queen, for that matter), or about communism. There seems to be something about the shift from a traditional partially-industrialized society to a post-industrial one which triggers this sort of great social upheaval, which manifests in part as new religious movements (labeled cults) and violent action (terrorism or guerrilla), which eventually get discredited and cease to be alternatives. In Japan, you had the 'rush hour of the gods' with many syncretic Buddhist groups and the Red Army (to name the most infamous one) with a last gasp in Aum Shinrikyo; in America, you had those but also Weathermen etc; in South Korea with its later development the process is still ongoing, with the cults take on a Protestant Christian form - the recently deceased cult leader associated with the Sewol Ferry disaster an interesting example - and the violence tends to be associated with North Korea (various assassinations or attempts come to mind).