bank details changed

For some reason my bank details with mtn has changed by its self without my concent. It changed to some capitec account. I have been with fnb for years. Now they charged me a penalty because of the return debit. How the hell does this happen. I would like to be heavaly compensated for this inconvenience. I have had it with mtn. I am already paying so much money every month. My bills come up to R600 per month. I dont feel valued and have been a very loyal customer. I want something done about this.i never requested to change my banking details.