SimpleBills gives you the option of using AutoPay and won't enable or disable this feature for you. First you should 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 have a past due balance. When an account is past due we attempt to collect a payment for utilities by using the payment method(s) saved on the account. This happens after repeated email reminders for an outstanding bill.