CSH,
There are two things going on here.
1) The switch to GIT over subversion. This is a no-brainer as "subversion sucks" says Linus.
2) The switch to github, over hosting our own GIT infrastructure. This is more debatable, but at the end of the day, most of the cool kids are on github and github has many very compelling features and UI to make development easy.
The hope is to make it easier for folks to fork and develop openfire, sending us back pull requests that are easy to merge and move forward with.
daryl