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Similarly, the person who has made the anonymous question has also put few tags on it. And if you search on users' profiles to see if these tags are included in their posted tags, you can find who asked the anonymous question (in a system with many users this is a lot of search, but still he could get uncovered). And the more unique these tags are, the closer you come to find the anonymous person.
PS. Sorry for posting this as an answer, I don't have enough karma to add it as a comment :)