Most likely Automatic Payment topped the wallet up. If the wallet you transferred from has it turned on, and the transfer takes that wallet below your threshold, the next automatic check charges the saved card, as with any other drop below the threshold. Coupon credit doesn't count toward the threshold, and the Transfer Credit confirmation doesn't warn you.
Before a large transfer: with that workspace selected, open Credit Balance, then Automatic Payment, and check or disable it.
Team wallets: the charge goes to the admin who last saved the team's auto-pay (see team auto-pay).
Where it shows: the top-up is a DEPOSIT row on Transactions.
Agent plans: a large transfer can also leave too little for an agent plan that renews from balance (see agent plan renewals).
Can't match a charge to a top-up? Contact support with its date and amount.