I don't think there is deliberate coordination.

It is more like the dancefloor effect (is it the official term?). When the dancefloor is empty, no one dares to take the first step, even if they want to, the someone finally steps in, then another, then another, etc... Dancing becomes more and more acceptable as the dancefloor fills up. Past a certain point, the opposite happens, it becomes unacceptable not to dance, so people step in even if they don't really want to.

I think the firms do not want to be held responsible for hosting discussions that lead to violence during the transition of power. January 6th made organized violence look like a more real possibility.

It's more likely a domino effect.

In other words, they've all wanted to do it for a long time, and now there's a social momentum that allows them to do it without having been the first firm to do it.

It's just optics.

As I understand you mean a hidden kind of coordination, not the obvious one, like "look at the news, they banned Trump, good idea, don't you think? shouldn't we do the same?". Why the hypothesis of such a coordination seems to be requisite? The obvious one explains facts nicely, I believe. I feel not the trace of a cognitive dissonance remained, so why to invent more complex explanations?