This was likely because the coins had not been moved in such a long time. The longer it has been since the last time a coin has been spent, the higher priority a TX that contains the subject coins. The TX was also very small in terms of the size on the blockchain.
No large amounts of Bitcoins are considered High priority transactions
While their is a relation to coin-age these type of transactions are large and significant so there is no need to pay a transaction fee.