The only person who can close a question id the original person who asked it.
The "Close this question" facility was not working recently,but I assume it is now back?
Why do they close it.
Several reasons:~
They are getting aggro/abuse over the question
They feel that they have enough answers.
The thread has been "hi~jacked" by other ABer's for a reason that is not answering the question.
Sorry Kakuro1,
You are confusing "closed" with "deleted" questions.
AB Ed can (and does) delete questions,they do not close questions.
Only the poster can "close" a question.
The difference?
A "closed" question can still be seen and previous answers read,but no more answers can be added to it.
A "deleted" question is removed entirely,along with any answers.