WHY NEDBANK CHARGESTHEIR CLIENTS ACCOUNT WITH BOUNCED DEBITS

I went to Nedbank tsakane branch on the 29 May 2018 after I realized that I was charged the total of R720 on my cheque account. I ask for clarity from enquiries and they failed to give me the answers they just told me to call Nedbank magtape for explanation, then I did so. Nedbank magtape said that total of R720 was charged because there were debit orders that bounced on my account due to insufficient funds and I explained to the lady by the name of yonelwa who was assisting me that I didn't authorized those people who debited the a total of R356 in my account. ive ask her to block those people from deducting on my account because I don't know the company, yonelwa(from magtape) told me that she cant I just have to call the company that was debiting and tell them to stop debits because I haven't authorized them to debit on my account. then magtape refered me back to the branch to lay a dispute to enquiries that those people were not suppose to be have authorized to debit on my account and may be they can remove that total of R720 that Nedbank charged on my account. i went back to enquiries at tsakane branch and i was told that there's nothing they can do because thats how my account is charging and there's nothing they can do because they don't have anything to prove that i haven't authorized the people who their debit bounced and thy further said Nedbank magtape is the one responsible to assist me in this case not them.

my worry is that why is the back charging me if the debit order bounced, because according to my understanding the people who were suppose to charge me is the once who debited and bounce. secondly why am i charged such R720 by Nedbank while the debit that bounce was R356.