"What do they want? Coffee?"
Translation:Mitä he haluavat? Kahviako?
I believe it's because the sentence follows the same questioning form as the "is it..." sentences.
In my head it's similar to the "Is that Joni? - Onko tuo Joni?" type of question. In this example it would be: "What do they want? (Is it some) Coffee (that they want)?" I believe it's because of this reason that the kahvi received both the partitive suffix (some coffee) and the question suffix.
From my observations ´Mikä´ goes only along with the verb ´to be´ e.g. Mikä tämä on ? Mikä sinun nimesi on ?
Moreover, ´mitä´ is in partitve in here, because you ask for some food/drink stuff and the answer/question that follows is ´some coffee?´ as ´kahvia´- is partitive from ´kahvi´.