What evidence is there that the companies are colluding? Are there communication logs where they all conversed and decided to pull ads? Is there any evidence at all that the companies had any interaction with each other about this and made a unifying decision to cancel their ads?

Collusion requires entities to work together to achieve a mutual goal. Otherwise, it’s just a coincidence of timing.

Capitalism has built in rules against market manipulation and monopolies

It most assuredly does not. Addressing these externalities is the responsibility of government.

So, are you suggesting regulation of the market?

Your definition of capitalism in this argument is simply a no true scotsman: https://en.wikipedia.org/wiki/No_true_Scotsman.

I think you might be having difficulty grasping the idea that people have marketing budgets and if say the ceo of a company you advertise on very publicly endorses hate speech it does create a brand management problem.

You want your products to not be associated with things like, say, racism, which are kind of “yucky” to a lot of people.

As a result you might refocus spending. If a bunch of people do this at once this doesn’t mean there’s collusion. For example, during a thunderstorm you might see less people outside. This isn’t because they all colluding - people don’t like being struck by lightning. Similarly, companies don’t want their brands to be “yucky” to the average consumer and often its just a matter of moving the ad spending to another platform without the baggage.

You could ONLY limit this effect by banning advertising entirely.

It’s not manipulation to say “we’re leaving because you did this thing and won’t be back until you don’t do this thing.” This is simply the market forces articulating their preferences.

If I stop buying a company’s products because I disagree with the direction it’s going, I am voting with my wallet, not manipulating the company.

Really? Because I’ve been repeatedly told by libertarian types (not socialists or communists) that any government regulation is not capitalism.

You’re free to disagree with them, but then I’m going to ask what your definition of capitalism is that assumes this regulation (not just allowing it, but mandating it).