The videos were outsourced, see above. Also with a sufficiently well designed physics simulation, something like a tree explosion is pretty easy. Take a look at some of the new Unreal Engine demos, or other tech demos that feature level/object destruction.
Also, take a look at this video from LucasArts
. It shows how creating good CGI is becoming easier and easier. Something like the trailers however would still take a good deal of work, and I'm not sure it would be easy enough yet to put out that quality quick enough to sustain a TV show.