why is the cook at the town angry when the soldiers how up to eat?

The cook is angry because he cooked for 150 men, and only a few showed up for dinner. Some were changing, but many were dead.

"They won't be fed by you to-day. They're either in the dressing-station or pushing up daisies." The cook was quite disconcerted as the facts dawned on him. He was staggered. "And I have cooked for one hundred and fifty men--"

dressing station is referring to hospital not physically dressing (dressing wounds)