bank account name was change witout any notoficati

For many years now my bank account name had been in my married surname - properly FICA'd with certified marriage certificate on file. Suddenly this month my surname was changed without any notification and without my permission. Who has authorized FNB to change my details? My security has been compromised by FNB. I expect a reasonable explanation to restore my faith in the credibility of FNB's security measures which are suppose to protect their Customers.