I think it's partly a reaction to how Corbyn was treated. You see the Peter Mandleson quote a lot about working every day to stop to Corbyn - but then it's used as justification to do the same with Starmer. That's something I find frustrating as I campaigned for Corbyn positively (even when I was being abused on the street, which happened on multiple occasions), and is more than a little hypocritical.
I think a lot of the social media "influencers" who got a higher profile under Corbyn (as with the Agitator on the previous page), are people who perhaps joined Labour only under Corbyn, so would have long been on the outside. The difference is that with social media, they've taken a lot of people with them, and so you have a much bigger group of disillusioned people.
The final nail is probably the leadership election. The party membership was still very pro-Corbyn, and anyone who wanted to be elected leader, had to be careful about that. He's obviously since ditched a lot of policies, although the fact that he became leader just one month before Covid, and prior to Truss and Ukraine, is clearly the reason for much of the financial scaling back. I can understand the anger over some of the other things he said, but much of that is the sad reality of politics. I'd suggest that as Labour members (and I'm one), we look at Tory members, and ask if we're really the best people to choose a candidate that is going to appeal to a broad enough coalition of people, to actually get elected.