If you are going to live, you might as well live large. And what better way to show off living large than to work on two of the biggest gaming franchises of the past 10 years? But of course to work on either--you have to roll exclusively with one or the other: http://www.gamepro.com/article/news/...fects-to-halo/
It doesn't seem like there was any ill-will or bad blood when Ryan left so it's all good where he's stands. As a matter of fact, it's like he's getting the best of both worlds because not very many people(if any at all) get to work on the top marque franchises for two rival companies.