Insubordination is a perfectly logical and legal reason to fire an employee. But juries will be suspicious if it looks like one of your supervisors "set up" the employee to give you a reason to terminate.
Too often, as in the following case, a manager who is looking to fire a "troublemaker" will egg that person on until he or she snaps, then fire the employee for insubordination.
Instead, take your time, build a solid documentary case based on measurable standards and follow your internal rules. Double-check that you treat all employees to the same scrutiny. Then, and only then, should you act.
Recent case: Rosemary Sylvester joined three female co-workers in filing a written sexual harassment complaint about the organization's CEO. Sylvester had an exemplary evaluation history, while the others had poor work records.
Days later, the organization's board decided to fire the poor performers, based on the assessment of one board member (a lawyer) that those women were using the letter to forestall discipline.
The board decided it couldn't fire Sylvester because of her good performance, but it left it up to her supervisor to fire her if she reacted negatively to the other firings.
The next day, Sylvester's supervisor told her the news. Predictably, she became angry. Also predictably, the supervisor fired her for that "insubordination." She sued, alleging retaliation, and the 7th Circuit sent her case to trial. (Sylvester v. SOS Children's Villages, No. 05-4219, 7th Cir., 2006)
Final tip: Don't rely on employment advice from a board member who happens to be an attorney. He or she hardly looks like a neutral adviser.