There are several reasons. They might not like their username anymore, they might like their new one better, or both. They may wish to not be found anymore but still be active. They could be changing...
For registered users only, there is a thread in the Community Forum titled "Suggestion Box". That is where users can suggest new features that will be considered in the future.
WikiAnswers is always making changes to improve the quality of the site and to allow the users to contribute more efficiently and easily. Oftentimes, there are minor changes that happen in...