I have identified several different mechanisms for the endless movement left.
Entryism, the conspiratorial takeover of organizations by other organizations. This is exemplified by Acorn, which has a single headquarters, but a hundred organization names, the residue of all the many organizations they have taken over.
Phariseeism: The Puritans were holier than thou, their successors holier than Jesus. “I am holier than thou, therefore you should obey me.”
The madness of crowds, the tendency of consensus to go horribly wrong
And metastatic apostacy, the tendency of official belief systems to mutate to greater virulence.
All of these tend in practice to be the same thing. Greater virulence is apt to be the same thing as greater holiness, greater holiness is amplified by the madness of crowds, and the entryist organization, being holier than the entered organization, takes advantage of the madness of crowds.
Thus in a single movement left, for example entryists taking over the the Science Fiction Writer’s Association, one observes entryism, the entryists are pharisees, the process of entry took advantage of consensus decision making, and the progressive pieties uttered to justify the takeover reflect that the official belief system is now more virulent than it used to be. Each of these mechanisms was involved.