In All Quiet on the Western Front, how do all of Paul's squad members die?
The most elemental answer here is that Paul's squad dies as a result of war. Part of what makes Remarque's work so powerful is that it demonstrates the sheer terror of war, and how the only plausible end result of war is death and pain. The narrative that Paul renders is one where the "lost generation" of youth in the war can only experience pain, loss, and estrangement:
And so it would seem. We had as yet taken no root. The war swept us away. For the others, the older men, it is but an interruption. They are able to think beyond it. We, however, have been gripped by it and do not know what the end may be. We know only that in some strange and melancholy way we have become a waste land.
The "waste land" that has laid its claim on the young people who serve in the war is the primary way in which all of Paul's squad members die.
The "waste land" that Paul uses to describe what happened to his squad members is reflective of how they die. They do not die in a glorious manner. Rather, they die because of the terrors of war, and almost as a testament to the sad absurdity of modern warfare. Muller, Kat, and Kemmerich all die as a result of lacking proper medical care to their various wounds on the front. Their deaths are indicative of how soldiers die on the World War I battlefield, being denied even basic medical care. Behm is killed in No Man's Land, a part of the sad death count in a piece of land that no one claims as their own. Westhus dies from a wound in the back that leaves his lung pulsing in daylight. Hans dies as a result of a direct hit, not even having a chance to survive as his body flies into pieces. Martens is rendered useless on the battlefield in the loss of his legs, while Meyer, Hammerling, Byer, and Detering are all killed in battle. Tjaden's whereabouts are unknown, while Paul is killed from a stray bullet.
These deaths are not glorious in nature. They are not triumphant statements of a Classical notion of war fighting. Rather, the deaths of Paul's squad members reflect the horrific nature of modern war, an exercise in savage brutality that renders people as only statistics and numbers which add to a rising casualty count. These deaths only add to the "waste land" condition of modern war.