This is pure speculation because I don’t know what happened here but my best guess is that this is money that was kept too long in an authorization hold. If this is the case, then while they would have seen a negative balance, they would not have been charged overdraft fees on that amount as overdraft fees are only charged on non-pending transactions that have actually cleared (this is an FCA requirement).
The 8% is quite common as the interest rate for where a bank has made a mistake and either held money or charged (by any error) and later refunded fees for any reason.