FNB Robbing Customers

When I checked my app today, I saw a negative balance on my account with a transaction #Item unpaid 1 for R150. I saw this the previous month and it was 2 Items unpaid for R200 and I thought it was a bug on my app. I want to understand what’s going on and why can’t I reverse this transaction? This is the reason why I’m considering taking my business to a different bank. FNB is robbing their customers in daylight!