At-will employment says you can be fired any time for any reason except reasons that are explicitly illegal. This would include, for instance, discrimination against a protected class; anything covered by whistleblower laws (FCPA, all the relevant Dodd-Frank whistleblower provisions); or for organizing a union. It doesn't appear to me that the employees in this case have a slam dunk legal case, given that their allegations mostly don't seem to line up with whistleblower protection laws and while they were collectively speaking as employees, they were not formally unionizing. There are likely other ways to pretextually fire people engaging in protected activities anyway. But the question isn't crazy, this is certainly adjacent to the kind of territory that could have some legal protection.