Historian Charles Tilly says that certain generations pioneer “Repertoires of Contention” – ways of asserting themselves of engaging politically.
I suggest that the individualism-collectivism pendulum swings between repertoires of contention, with the individualism or collectivism ideologies following from these repertoires. For example:
The Progressive Generation (born 1843-1859) – in midlife (after 1890), innovated regulation, muckraking, and institutional reform. Institutional reform and regulation are particularly collectivist in nature.
Boomers (born 1846-1864) – engaged in sit-ins, freedom rides, campus free-speech actions (pioneered first by Silents, but taken up more broadely by Boomers), mass marches, campus occupations. These methods of contention promote individualistic ideology in that individuals freely participate in these action.
Millennials (born 1981-1996) – promoted digital activism, hashtag campaigns, and to some extent cancel culture. Cancel culture in particular is collectivist.
My suggestion is that the pendulum swings primarily because the repertoires of contention from older generations become ineffective. Individualistic repertoires are ineffective when a few individuals pull ahead of everyone else, becoming super-wealthy. And collectivistic repertoires become undesirable when groups arrive at a peace, perhaps with some groups dominating the geopolitical world, and the collective desire for such conflict is exhausted.
Nexus State Theory 