A couple of possibilities here.
If your second payment hit around rent time (a few business days before the 1st through the 7th of the month): we charge your card during that window so your bank account stays clear for your next rent payment right after. No bank pull during rent week.
If it hit outside that window: we'll automatically choose between your debit card and bank account to keep your payments running smoothly — we pick whichever is most likely to go through. This time that was your card, usually because it looked like the cleaner path compared with your bank account.
Want to choose the method going forward? Pay before your due date. Open the Split Pay app or tap "Manage Your Payment" in your reminder email, choose Pay Now, and pick the card or account you want.
Changing the method on the Pay Now screen only applies to that one payment. To change what your automatic payments use going forward, update your default payment method at splitpay.com/account.