This happens for the following two reasons:
1) You have leftover transactions that were processed under the check payment method. All transactions must pay out under the account information in place at the time they were received.
2) You did NOT previously set up a payment account but used the “Set Up Later” feature to accept registrations. This is our Deferred Setup method. All transactions processed while in Deferred Setup MUST be paid as a check. Transactions processed on your own payment account you will be paid according to that payment method and schedule.