Multiple former employees have apparently raised a concern about either your general management style or an alleged violation of a company policy. HR has likely investigated the matter, including talking to you and other relevant parties and gathering additional data such as your performance reviews, prior complaints against you, the history of turnover on your staff, etc.
Based on HR's investigation, the company may determine that corrective action is necessary (i.e., discipline up to and including discharge, performance coaching, job change). The fact that the complaint(s) came from employees who are no longer with the company does not negate the allegations. Investigative complaints commonly come from not just current employees but also customers, vendors, family members, ex-employees, and sometimes anonymous parties. If there are multiple people saying the same thing that makes allegations more credible unless they have a motive to hurt you.
As far as your rights are concerned, you can ask whether there is/was an investigation and what has been specifically alleged against you, but HR does not have to disclose it. It's not a court of law but rather a company's process. Therefore, your best bet is to tell your side to decision makers, listen to the feedback, and not even think about retaliating because that could make it so much worse.