Halliday & Matthiessen (2014: 103):
Thus in both kinds of interrogative clause the choice of a typical ‘unmarked’ thematic pattern is clearly motivated, since this pattern has evolved as the means of carrying the basic message of the clause. Hence there is a strong tendency for the speaker to choose the unmarked form, and not to override it by introducing a marked Theme out in front. But marked Themes do sometimes occur in interrogatives.