In some instances the question doesn't itself fit a particular tag, but the accepted answer/popular answers do. If tags are a useful guide for others then it could make sense to re-tag based on the answers as well as the questions?
For example, often the question might not specify the language, but then R based code examples are given in the answer - should these be tagged R?
Or the answer to the question might involve proposing the use of a particular statistical method, that would not be known to the question asker. So if the answer involves, try using X, should the question get tagged with X?