My BK does that too. And I can tell you from about 3 years of working in fast food myself, they do it to make sure drive-thru doesn't get backed up. If your food is a big order, or you order something they need to put through the fryer for a few minutes (Like chicken nuggets, patties etc.) that will take longer than the typical burgers because they don't have those pre-made in a meat chest already. So in case another vehicle does go through the drive-thru that maybe just orders a fountain drink and a cheeseburger, they can get those orders out of the way first so that the orders don't get backed up and workplace goes to hell.
how about they do orders on a first come, first served basis ---