I find that if you are in an category, 'ask a question' link will bring you to the ask page which category has already filled with where you came from.
It does really work on 'activity' & 'questions', but seems you are in 'unanswer' page, then you click some category link in this question list. You will go to 'unanswer' under this category. At this time, you click 'ask a question', the category won't fill.
Is this a bug?