It has been a gradual process and I have been watching it with dismay since I was young. "do your own thing" has morphed into "do OUR thing OR ELSE!"
My own explanation is that tribalism gradually replaced liberalism as the guiding force of the left as people embraced identity politics and the fierce need to be a member of a tribe has resulted in increasingly authoritarian attitudes.
When I was young (I'm 65) the right was more authoritarian and the left more libertarian. Today, the reverse is true. Anybody with half a brain should be able to realize that it is the left that has seized upon the corona virus as an excuse to stomp all over people's rights and not the right, yet the good little conformusts of the left just go along with it because they are profoundly illiberal little sheep who only know how to soldier on for their tribe.