why did you debit money after i cancelled the plan and on the wrong date

i have been using liquid capital for the past 2 years for service plan and I decided to cancel it on the 31st September and on the 1st of October they debited R545.92 which bounced and now I owe the bank R100 for that. Liquid Capital never debited on the 1st but to my surprise they did that this time after I cancelled the plan.