These are quite common - look it was mined very soon after a block had just been mined.
Issue is that after a block has been mined the mining pool has to get details of a new block - this involves verifying the last block was OK, removing all the transactions that were in the last block from the memory pool, select new ones, building the merkel tree and then the header to mine with. This takes some time (should be less than a second).
So during that second a lot of pools just create an empty block - this will always be valid - with the last block has as the previous block - some do not even validate the old block - especially as in this case as they mined it, and send that header to the miners to mine.
If they did not do this the miners would be idle for a short period of time every time there was a new block.
Mining an empty block for 12.5 btc is better than doing nothing...
You can see there was plenty in the mempool here:
http://statoshi.info/dashboard/db/memory-pool