Think of following pattern of events:
- User A asks question.
- User B answers this question, but don't upvote the question.
- User A upvotes User's B answer.
User B checks whether his answer had been upvoted and if and only if it looks like it was upvoted by the User A, only in that case up-votes the question itself.
By "looks like it was up-voted by the author of the question" I mean that there is such set of empirical rules which can help us to probably guess that kind of situation. If this sounds too theoretical to you, then just think of the case when user B up-votes if and only if his question is marked as accepted.
Should we consider such behavior a sort of shady transaction? I mean, user B actually does not even care about the quality of the question he is answering. This is only about a deal - up-vote my answer, and I'll up-vote you question too. In turn, user A who finds this deal fair also is not so strict about answers quality.