PayPal reviews all transactions to help sellers avoid risk. If they think that a payment might be risky or fraudulent, they'll hold it for review. The review process usually takes less than 24 hours, but sometimes can extend to 48 hours. Having a fully verified PayPal account might help avoid these holds, but it's not guaranteed. If PayPal feels that there may be a higher than normal level of risk associated, the transaction will be held for review. This is for protection of the buyer, seller, and the PayPal system.
You can read more about payment holds here.