Legal Question in Employment Law in California

While your expectations are reasonable from a business and employment standpoint, you have no legal right to them. Unless you have a written contract that requires you only be fired or demoted for just cause, and as long as they are not taking the action based on illegal discrimination such as race or gender, your employer has the right to make any changes they choose to your terms and conditions of employment, including duties, title, and pay (as long as they don't try to lower the pay for time already worked), or they can just fire you, for any reason or no reason at all.

And not as legal advice, but as a person who has been in the workforce as an employee and an employer since 1975, my reading between the lines is that either they did not find your management aspects of your work satisfactory, or they are really consolidating and needed fewer or different managers, which would suggest the company is not doing particularly well. In either case, it probably is a sign you should start looking for a better fit while you still have a job.