If you've got a Battle.net account, you might want to log in, especially if you haven't been on for a while. Various Reddit users have been reporting that they have been getting StarCraft II: Wings of Liberty for free over the last several days. There's been no official explanation given on the part of Blizzard, but theories have been flying around that the company is planning on making Wings of Liberty permanently free-to-play.
There's no proof behind these theories, other than the fact that the logic would appear to make sense. StarCraft II has fallen behind on the competitive gaming scene over the years to other games like League of Legends and DoTA 2. The obvious attraction with those games is that (other than being tons of fun), they are free to play for anyone.
It's worth noting that the giveaway does not include StarCraft II's expansions, Heart of the Swarm or Legacy of the Void, but giving away Wings of Liberty has the potential to tempt new StarCraft II players into buying the rest. As it stands, Wings of Liberty's official retail price on Battle.net is $19.99, but retailers with physical copies have gradually reduced the game's price over the years, with Walmart even carrying it for as little as $10.
Considering its age, Wings of Liberty on its own does not appear to have much value to retailers, and with the dwindling competitive player base amongst a growing expectation of free-to-play competitive games, it makes a ton of sense to make the game accessible to as many players as possible.