It is illegal to retaliate against workers for organizing or joining a union. Texas is an at-will employment state, which means it is legal to fire everyone for any reason that isn’t explicitly illegal (like sex- or race-based discrimination, or retaliation against union organizers) or even no reason at all.
So the dance company can fire everyone because it was Friday and the sky was blue, and it is not illegal. But they have said that all of the dancers violated some internal conduct policy, so a court will evaluate the policy and determine whether or not they were fired for an illegal reason. At-will employment law exists to make it extremely difficult to prove that anyone was fired illegally, but there is a lot of evidence in this case so there’s a chance.