Standard Bank VAF random installment increase

Standard bank just increased my VAF installments by over R400, I just got a mandate on my banking App to accept their Debi heck which I REJECTED with the hope that someone from Standard Bank will call to explain what this increase is about especially because the rates have been going down so an increase is a shock. I have never missed any payment so I do not understand what this is about. To make matters worse they have attempted to debit my account for the unauthorized amount so what is the use of sending a request to get authorisation yet you still debit. I need an explanation and a clause in my contract that states that you can increase whenever you feel like it with no communication.