One possible cause is that the seller has refunded you before your initial payment to them has cleared from your bank to PayPal.
When you sent your payment, the money is not instantly transferred from your bank account to the recipient’s PayPal account. Instead PayPal instantly credits the recipient's PayPal account on your behalf and simultaneously request the same amount from your bank account to reimburse ourselves.
It takes 5-7 business days for the money to arrive from your bank to PayPal (same timeframe and method as an eCheque in fact). If your Instant Bank Transfer is refunded, this refund cannot be completed until PayPal receives the money from your bank to cover the original payment we made. A ‘Temporary Hold’ is ‘Placed’ on your refund while your bank processes your initial transfer request. When PayPal receives the money from your bank, it will be credited to your PayPal balance.
This also applies when you cancel an unclaimed payment
