To fully appreciate this stylistic choice, it's important to return to the ending of the first stanza. The speaker is part of a group of soldiers who are weary from fighting. They march as if they are "asleep" back toward a point of "distant rest." They limp and many no longer have any boots. This stanza reflects the great "fatigue" which slows the men down, and there are several examples of caesura as well as hard stops at the ends of lines to ensure the lines are read slowly. Suddenly, everything changes as "gas-shells" land "softly" behind them.

The second stanza opens with the horrific realization of what is happening. The soldiers are being attacked by gas bombs. This was a fairly common wartime tactic in World War I which created sudden and horrific respiratory reactions, often leading to death. It was therefore important to quickly cover the nose and mouth with a gas mask to filter out these gasses before breathing in the chemicals.

This sense of frenzy is captured in the opening of the second stanza:

Gas! GAS! Quick, boys!

The soldiers are yelling to each other with urgency, indicated by the exclamation points. "GAS" is capitalized the second time to reflect the immediate need for everyone to hear these orders and to comply as quickly as possible. The voice of the speaker is louder because of the capitalization here, and the tone shifts from weary to panicked as the men quickly move to save themselves and each other.