Ok you need to really learn what Twitter is or just stop commenting on it. This is like explaining the internet to my grandparents
Here let me break it down for you:
Following John Cusack is a way to get a look at what both the "BeRnIe or DIE!!!!" crowd believes AND the legitimate mouthpieces and media types that support Bernie think. If you follow him he often retweets them. As such anyone who follows him (or follows someone who follows him or whoever else the algorithm decides) sees this. Now the people he is often retweeting on these matters are people like say Mehdi Hasan, Krystal Ball (who I havent seen comment on this), Peter Daou amongst others who are politicians, activists and media types for various print, online and TV sources. They say it, he retweets it and so on. I use him because he is a good example but honestly I bet if you just followed Krystal Ball or The Hill you would see most of it. The movement to destroy Biden because Bernie cant win legitimately is Putin-esque in its efficiency. (no I am not saying Putin has anything to do with it)
Now, if it was being only disseminated to the Berners I wouldnt much care. They would see it and create their bubble and we could ignoreit. The problem is many of these people are followed and follow more mainstream types. As such this stuff doesnt stay in the Berner bubble but goes viral. Twitter makes sure of it. That is why I see some nutjob Righties despite not following anyone of that ilk. I might follow someone who has retweeted James Woods defending Glenn Beck or something and there we go. (that is why The White House and President always end up in my feed)
tl;dr: John Cusack is just the guy that gives us the glimpse. There are dozens of media, politicians and activists out there using their cache and thousands of followers to try and tear the Democrats apart right now.