Selection is based on the information in your profile and answers that you’ve previously contributed. I am prompted to answer grammar questions, for example, because I have grammar as one of my interests. Lately I have been prompted to answer AnswerBag questions, also, because I’ve tried to be active in some AB questions (although obviously AB isn’t in my profile). And lastly if someone in your fluther has asked the question or answered it, it may show up in ‘Questions for You’ (I think).
Presumably the more time you spend on Fluther, the better it will get to know what kind of questions you want to answer.