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.
If the size of the TX (in terms of space on the blockchain) is small enough then there is no reason for the miners to force the sender to pay a fee. Also if the sender was somehow associated with a mining pool then they can simply have the pool confirm the TX when the pool finds it's next block.
