SimpleBills gives you the option of using AutoPay and won't enable or disable this feature for you. First, double check to see if AutoPay is enabled on your account. You can find this setting under the Payment Methods tab.
If AutoPay is disabled on your account but you were still charged, it is most likely because you had a past due balance. When an account is past due, a late payment draft is attempted by the payment method(s) saved on the account. This happens after repeated courtesy email reminders for an outstanding bill.