I've had a few payments from buyers end up in that same review and they have all cleared within 24hrs(so far)
Why exactly or what they are "reviewing" I have no idea.
I believe that can happen if the buyer is using a PayPal account where there is no bank account linked to it. There are some trades I've done where I was informed in advance by the (trusted) buyer that this would happen -- and sure enough it did get placed "under review" and then in under 24 hours after the transaction the funds were released.
I know that SpendBitcoins had the problem recently where they had to re-verify identity, so perhaps PayPal treated it similarly as not having a linked account.