dunno, just seems to be the default behavior.. the phpBB forum I set up a few months ago is the same way... although when I look at phpBB community forum, it is set up as oldest first.
I looked at their help files and I couldn't see any mention of it, and the search option on the help forum is not working..

..my guess is it's not so much a bug as an oversight and is probably something that should be stored on the cookie as part of your preferences.
Personally newest first kinda makes sense to me as a regular.. saves having to scoll to the bottom of every thread if I've already read most of it.