New York Times

From what I've read...

There's the potential for a maybe-legit case here. Part of anti-trust laws include restrictions from competitors collaborating together, to gain some kind of advantage in the market. Price fixing would be a good example. A large group of advertisers collaborating on which clients they will or won't do business with does seem like it could fit the bill.

My guess is it'll come down to WHY they singled out Twitter for blacklisting. Does it give these advertisers some advantage over competitors in the market? Or allow them to exploit consumers in some way? Or is it maybe because the CEO of the company made false promises, and then publicly told them to go fuck themselves?