It's all about space. In theory, it would be best if there were a pad relatively near each end of each rwy and that aircraft would pull through that pad on their way to takeoff clearance. Aircraft would be sequenced for de-icing so that it is done once and only as they head out.

That's not possible at many airports and it creates the situation which ORD provided yesterday. By the time a deiced aircraft makes it into position, it needs another treatment.