Companies can cancel your vested equity for any reason. Read your employment contract carefully. For example, most RSU grants have a 7 year expiration. Even for shares that are vested, regardless of whether you leave the company or not, if 7 years have elapsed since they were granted, they are now worthless.
Once vested, RSUs are the same as regular stock purchased through the market. The company cannot claw them back, nor do they "expire".