For the Dominion, the point of divergence was the mid 60s with the election of socialist Pierre Trudeau and the replacement of the Red Ensign. The Trudeau government willingly sided with Communist regimes like Cuba (whose leader would give us Justin), implemented the social programs modern Canadians see as sacrosanct, etc. The adoption of the Maple Leaf flag erased Canada's identity as the brightest jewel in the British Imperal crowm and replaced it with the modern "We like maple syrup and are better than America, eh" identity she now presents to the world.
For Britain and western Europe, one of the aspects was the horrors of two world wars led people to become more atheistic ("If God real, why bad thing happen?"), and thus more accepting of socialist policies.
For America, it was the counterculture of the 1960s, which was promoted by the KGB.
You mentioned the fall of the Iron Curtain as a factor, and I would agree. With the major Communist power out of the picture, the threat of Communism became less real and concerns for same were seen as quaint throwbacks to the "McCarthyist" past.