ODST was a rare situation where Bungie had wanted to make a DLC side story, and Microsoft wanted more Halo games to sell to consumers. You have to remember that for a long time Halo 3 had huge hype and was a big part of Microsoft's Live sales. Microsoft forced ODST as a $60 stand alone game, instead of a $20 DLC. To compensate for what they believed to be an over-inflated price, Bungie worked out that all of the DLC would be included extra on a separate disc. Still Halo 3's multiplayer, to not divide the community, but all of the multiplayer maps as an added value to ODST's price. Gears of War isn't that series, though.
This is just a quick cash grab while the resources are available. It's smart. The next generation of consoles is impending, and with the hype building for the Wii U, it's clear both Microsoft and Sony will have to consider when to launch their next systems, especially if their current systems will be limiting development of 3rd party games, possibly costing them support from certain game studios then.
Gears of War 3 sold well, but Epic's abandoned support of the game, they've released all of its DLC, and thus have hit the peak of their money grab for that game. Releasing a new game in the series, that probably runs off of the Gears of War 3 engine, and is just slightly modified, would be a new sale for them to profit from. I would not be surprised if this was sparked by the sale of RAAM's Shadow, where a 'classic' styled campaign DLC probably sold very well, and the critical reaction of the Lambent perhaps interested Epic in attempting to appease fans of the series who want more, but also really just get more money quickly from a game engine that is already setup, even if People Can Fly are possibly handling this.
Clearly next generation will require understanding the new system(s), building resources, and testing launch titles. Throwing out a quick prequel for this current generation console that runs off of the last games engine gets them easy money, and also is easy to develop. Plus, Gears of War 3 isn't even a year old, so announcing this is keeps Gears of War relevant, though also dooms the interest in Gears of War 3 (but, as noted, the maximum profit from that game has peaked, so losing it was probably a worthy sacrifice in Epic's mind).
This isn't good news or bad news, just more proof that I was right that they've already abandoned Gears of War 3, not even a year after it's release. Epic has changed...
Also, noting this, I laughed when people mentioned the idea of dedicated servers existing in Gears of War 3 when this new title in the series releases. From all we've seen with Gears of War 3 support so far, I'd greatly expect for the dedicated servers support to diminish by the year anniversary of Gears of War 3, with Epic probably claiming that resources and spending on servers are needed elsewhere for future titles and next generation development.