Does an accountant legally have to release my files for tax purposes and why?

The CPA is required to return all of YOUR records upon request. YOUR records are items that you gave the CPA. A CPA isn't required to give you THEIR records, documents that they created in the course of working on your taxes. However, a CPA (or anyone else that does tax work) is required to give you all necessary documents to complete your taxes regardless of who owns the documents. If the CPA is charging for work that wasn't completed and/or refusing to return your records, you can file a complaint with the state accountancy board and/or the CPA society. That gives you a lot of leverage in this transaction. I would recommend that you get your new CPA to handle this for you.