I was a wee bit surprised by this, especially since Microsoft has been so busy lately, but this acquisition makes a ton of sense. Like Halo, Gears of War is a flagship game on which originally drove console sales for rapid multiplayer fans. Gaining control of the franchise gives Microsoft an advantage in coordinating their marketing efforts for their consoles. See full announcement from Microsoft.
I am now trying to figure out what Epic games (the original owner) gets out of the deal besides a wad of cash. My assumption is that Epic is transitioning from a game company to one who primarily focuses on licensing their Unreal game engine to third parties. The one wrinkle in this is that they announced Fortnite in 2011 for release sometime this year. 2011 was a long time ago in video games years though so it could be that Epic has decided to strategically change directions since then.