As has often been the case, it was created by a 17 year old teenager with nothing better to do; in fact the guy seems quite talented as he has created his own Twitter-like service (which I'm not going to link - seeing as that was what the worm was doing) - which I suppose is not very easy.
The thing that is mind-blogging is the reason he gave: apart from being "bored", and the standard 'exposing security problems', he said that he wanted to
"promot[e] myself or my website." (bnonews.com)
Think about it: you want to attract people away from Twitter, what is a good way to do this?
1. Provide a better service, do promotions, get media/blog coverage, convince Microsoft to buy you out, etc. etc?
2. Hijack their accounts and piss them off forever?