What special accomodation has to be made to save Bombur from the dragon? why?

Bombur and Bofur were brought up from the valley by ropes. Just before Smaug attacks, 

Roused by these words they were just about to creep inside the tunnel when Bifur gave a cry: "My cousins! Bombur and Bofur - we have forgotten them, they are down in the valley!" "They will be slain, and all our ponies too, and all out stores lost," moaned the others. "We can do nothing." "Nonsense!" said Thorin, recovering his dignity. "We cannot leave them. Get inside Mr. Baggins and Balin, and you two Fili and Kili-the dragon shan't have all of us. Now you others, where are the ropes? Be quick!"....Up came Bofur, and still all was safe. Up came Bombur, puffing and blowing while the ropes creaked, and still all was safe. Ch. 12