The post does not include a reason for the discontinuation, but simply says SOE "determined ... to discontinue game services" with partner Wizards of the Coast. SOE will retain all of the game's code and data and will not license an emulator or fan-operated server.
The game's forums and websites will close as well. No new card packs will be sold past Nov. 18, 2013, though all current decks players have will be accessible through the game's end. Station Cash will still be usable in other SOE games.
Magic: The Gathering - Tactics launched in 2011. For more on the game's discontinuation, you can read the forum post.