That's where there's a dispute between the union and company A, so the union not only pulls company A's workers out on strike, but also company B and C's.
It's a tactic most often used just to be assholes.
Note that traditionally company B and C aren't just arbitrary as the parent kinda makes it seem. It would usually be something like restaurant workers at A are on strike, and truck drivers working for B refuse to deliver to A in solidarity with them. It is totally illegal under Taft-Hartley, which certainly isn't only about constraining big business.
