They went from one set of beliefs to another. Whether mine, my girlfriend's family in HS, my SIL's.
It always went the same way.
Somebody in charge decided to change something that was traditional. A lot of people didn't care, some were against it, some for it. Those "for" tradition lost and the rules changed.
Not only were the rules changed, but suddenly it was *wrong* to do what they'd been doing for years, what the church hierarchy had said was The One True Way. Suddenly last year's One True Way was actually (and always had been) a Highway to Hell. And the dictum was "conform or be cast out."
When the traditionalists, not-even-late-adopters of the change complained--when mere tolerance would have been sufficient--were described as having started the altercation and were soundly condemned for it. Maybe some subgroup was taken to be representative of the entire group, maybe their views for letting both practices coexist were simply too much "temptation to sin", maybe something else. A schism is born.
It always struck me as a wonderful bit of Stalinesque agitprop to say that the change was uncontroversial when obviously it had been, and that those controverting it were suddenly those launching the attack. Sadly, it never failed and those who held to last year's One True Way were always the losers and were kicked out of the organization that often they were the most zealous in building and most devoted to its precepts.