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.
I am not saying they are high priority transactions today, I am saying that if the TX fee was based on the amount being transferred then large transactions would receive higher priority because the miners would receive a higher fee for the transaction.
It is important to note that the individual miners (or pools when a pool finds a block) are the ones who set the policy as to which TXs are confirmed in a found block.