Imagine you request a medical time off under FMLA, your manager mistakenly believes that you are not entitled to it because somehow he/she determined that your medical condition is not serious enough to qualify or even worse - the employer thinks you are faking it. You then give your that manager a peace of your mind, telling them that they are not good and they are incompetent. Perhaps you raise your voice and you also use some inappropriate language. At the same time, you go to his/her superior and complain about denial of your rights. You might even send a nasty e-mail or two on top, to other managers to tell them how unhappy you are about your bosses ignorance. These actions give your employer an independent, and perfectly legal reason to terminate you - being rude, verbally violent and insubordination.
Instead, you should always act like a "politician" , so to speak. You should never lose your cool or be rude or inappropriate with your boss. Most of the times, employee's right are violated not on person but because the employer simply doesn't know better or because the company has to deal with certain financial or productivity pressures that push them in the wrong directly legally. This is not to justify their actions but to make you realize that there is absolutely no point and no benefit in raising your voice or becoming aggressive toward your employer.
Further, when you remain civil at all times, and the employer does violate the law by discriminating or retaliating against you, your potential legal case will be much stronger, as the employer will not have the "insubordination" excuse for retaliating or firing you. The more calmly you act in any legal proceeding, including your deposition or your testimony in court, the more likely you are to do better, and in large part because people who are calm are also more believable.