[Update: A bunch of people have commented that this was not actually a bird getting chucked into the engine, but an explosive charge that dislodged a blade inside the engine. That gives you a more dramatic piece of footage than otherwise, but the results are similar.]
Remember, the fans inside a jet spin very rapidly, and are under a fantastic amount of stress. Poke them hard enough, and kablam.
My first thought was, "Why this doesn't happen more often?" but then I realized that birds actually don't take up a lot of space compared to the volume of space a plane uses, and probably stay away from airports because of noise. I suspect that's a good thing. [Update: BABloggee "Grey Wolf" informs me it is indeed a problem. The FAA spends as much as half a billion dollars per year on this! Wow.]