It shouldn't really be much of a surprise, since this happens pretty regularly with super successful game development studios, especially those that see a meteoric rise like Rovio has had over the past couple of years. Angry Birds has been a worldwide cultural phenomenon the likes of which we probably won't see again any time soon, and now two of the leads are breaking off to start their own development studio.
The lead server architect for the original Angry Birds, Antti Stén, and Rovio's lead artist, Tuomas Erikoinen have left the company to start their own game studio called Boomlagoon, which aims to make "convivial and intriguing games". This is a pretty solid duo given that Stén was responsible for building the infrastructure for Angry Birds, which has scaled to meet the demand for millions of users; and, Erikoinen was the lead artist for all Angry Birds games except Angry Birds Space, where he was a supervisor.
The two do plan to hire more people to fill out the team, and are planning to make games for every platform, though they say they will start will HTML5 games.
Rovio did say that it is working on a new game
divorced from the Angry Birds world, but we can certainly understand Stén and Erikoinen getting tired of everything being Angry Birds and wanting to do something really new.