There is a tendency amongst those on the political extremes to:
Silence opposing voices; Push for, and sometimes implement the policing and criminalization of certain speech; Root out 'enemies' through witch-hunts and humiliation rituals or 'struggle sessions'; Enforce a conformity of ideas and opinions through the threat of ostracization, humiliation or violence. Instill a culture of fear around speech or activities contrary to the 'party line.' Amongst other things.
There are always consequences, some more productive that others. Note that I am not claiming that progressive politics are authoritarian, only that there are authoritarian tendencies on the far left end of the progressive spectrum. I would probably class myself as progressive. Green/Labour left is probably closer.
Where one might say 'overly zealous' another might say 'authoritarian.'