I've seen new users who choose a name and ask a question, revert to USERxxx after a few days. What causes this?
This might be one reason: My profile just went blank? ... quoting @@JeffAtwood:
We are no longer inferring usernames from openids.
If you want a username you must set one.
This is by design and will remain this way from this point onward.
... However, that was back in September, 2010. Did you spot this behavior just recently, or was it an older question you were looking at?