Why does Ponyboy pass out after the fire?

Ponyboy passes out after the church fire in The Outsiders because Dally hits him on the back hard enough to accidentally knock him unconscious while putting out flames that have caught on Ponyboy’s back.

The fire happens when Ponyboy and Johnny return to the abandoned church and find it burning with children trapped inside. They rush in, lift the children out through a window, and keep going even after Dally warns them that the roof is about to collapse. Johnny saves Ponyboy by knocking him through the window just before the roof caves in, but Ponyboy hears Johnny scream behind him.

When Ponyboy wakes up in the ambulance, Jerry Wood explains what happened: Ponyboy’s back had caught fire, and Dally knocked him out while smothering it. Ponyboy’s injuries turn out to be relatively minor burns, while Dally is also burned and Johnny is critically injured with a broken back.

Ponyboy does not faint simply from fear or exhaustion. He loses consciousness because Dally physically knocks him out in the process of saving him. The moment fits the larger pattern of The Outsiders: boys labeled as "hoods" repeatedly risk themselves to protect others, and their actions matter more than the gang labels placed on them.