Happened to me once - about 9 montths after the sale.  I contacted the buyer for clarification and he told me that his card had been fraudently used and his bank was sending charge back requests on many traqnsactions - even thiose that were legit.  To cut a long story short, paypal had no option but to reverse the payment back to the bank, but my buyer immediately authorised a new payment so all was good.  Make sure you communicate with the buyer before you do anything as in many chargebacks, its the banks and not the buyer who do it. 

Also - if you get paypal to contest it, they will hit you with a fee - I think its $15