Honestly, I blame the Religious Right and their prominence in American politics during the last two decades of the 20th Century and the first few years of the 2000's decade. These guys were the moral authoritarians of their time, but have largely been supplanted by the SJW's as the new moral guardians, at least on the national level.
Meet the new boss, same as the old boss.
The Republican Party in particular was dominated by the Moral Majority during this time, and the fundamentalists were also a major voice in the culture of the Southern states, Appalachia, and the rural Midwest.
During the 1980's and early 1990's, you had the Satanic Panic, which originated in the Evangelical movement of the late 1970's and early 1980's, but was for a long while, a bipartisan effort between the Right and the Left, mainly due to misinformation like the book "Michelle Remembers" combined with actual facts such groundbreaking studies that showed how utterly damaging sexual abuse was to minors, and all of this was right after the mass counter-culture movements of the 1960's and 1970's and the end of the Sexual Revolution, and was concurrent with the AIDS epidemic that cast a very dark shadow over the "free love" era of the 1970's (especially in regards to the gay community)
So naturally, you had a unified front with the televangelists on the right and people like Tipper Gore and Joe Lieberman on the left, and with it came mass hysteria over largely innocuous things popular with the youth of the time like Dungeons & Dragons, Heavy Metal music, and Halloween celebrations (which resulted in the inevitable bans and censorship).
By the early 90's, the Satanic Panic died down as a national phenomenon as the Religious Right and the liberals stopped working together, especially after the controversies over the West Memphis Three.
However, in traditionally conservative areas such as the Bible Belt states, the Satanic Panic endured for most of the 1990's, still targeting Halloween, rock music, and RPG's (only now it was Vampire: The Masquerade instead of D&D) but also adding video games, cartoons, and Harry Potter books to the list.
The Left did work together with the Religious Right in their crusade against violent video games once more in the late 1990's and early 2000's after Columbine.
This kept going in the Bible Belt until 2001, with 9/11 being such a massive shock to our collective cultural conscience that even the most hardcore fundies stopped worrying about hypothetical Satanists and witches as the very real threat of Islamic terrorism became the new source of fear and panic.
Most Millennials were born between the mid-1980's and the mid-to-late 1990's, which means that for most of their childhood, the moral guardians who were ruining their fun and cramping their style were mostly hardline religious conservatives, especially if you lived in the South or the rural Midwest.
Combine this with many Millennials entering their tween and teen years during the rather conservative Bush years that gave us two failed military quagmires, the creation of the modern surveillance state via bills such as the Patriot Act, and ended with the Great Recession, and you had a generation that felt alienated by what they perceived as traditional values.
Ironically, in their determination to rebel against the old moral guardians of their childhood and adolescence, the SJW's ended up becoming the new moral guardians and were far worse and far more effective in every way than the Religious Right could have ever hoped to be, the only real difference was that the old moral guardians were.
The Frankfurt School/KGB gay ops in our colleges and media only added fuel to the fire and simply gave the SJW's an edge that the Religious Right never had. They could shout "Fuck you, Dad!" on an unprecedented scale, especially with the rise of social media.
The Moral Majority accidentally caused a cultural pandemic. The only thing that the post-modernists, the so-called "Frankfurt School", and the college Marxists did was drive the monkey to the airport.
Meet the new boss, same as the old boss.
TL;DR=I honestly blame the Religious Right for the rise of SJW's in American culture more than anyone else