Perhaps Aaron has some genetic mutation or special ability that he was unaware of until the explosion. This ability could have allowed him to withstand the force and heat of the explosion.
Maybe Aaron discovers he has some new superpower as a result of surviving the explosion. For example, he could have the ability to sense danger before it happens, which would be a really cool development in the fanfic.
The man could have been caught by some structures on the building during the fall, which reduced his falling speed. Also, his body might have a special physical condition that allows him to withstand greater impact.
One reason could be his superior knowledge of magic. He might have found a way to counter the power of the Infinity Stones using his vast understanding of the mystic arts. For instance, he could have created a spell that made him invisible or immune to the effects of the snap.
It might be that in that particular fanfiction, the author was exploring different themes such as sacrifice on a grander scale. Harry Potter sacrificing himself to save the entire wizarding world in a more permanent way than in the original story. For example, perhaps the threat in the fanfiction was so great that his death was seen as the only solution to end the chaos and darkness.
He had some sort of special shield or protective gear that shielded him from the blast.
One possible cause could be a conflict between rival gangs. In pulp fiction, there are often violent power struggles. Maybe a deal gone wrong, like a drug deal or a heist, led to an explosion as a form of retaliation or to cover up evidence.
Poison or some sort of harmful jutsu effect. Maybe an enemy uses a special poison or a jutsu that slowly weakens Naruto's body. In the fanfic, this could be a new and unique type of attack that he has not encountered before, and his body is not able to withstand it, resulting in fainting.
I'm not sure exactly as this seems like a very specific and rather disturbing or inappropriate concept. It might be some form of fictional writing within a certain niche that involves gay characters and a very extreme and inappropriate situation, but this is not a common or acceptable mainstream topic.
One reason could be to save him. Maybe Harry is injured during a battle or some sort of magical accident, and Snape, despite their differences, steps in to carry him to safety. For example, if Harry is hit by a dark curse and Snape, with his knowledge of dark magic, knows he needs to get Harry to the infirmary quickly.
One possible reason could be that he is influenced by Orochimaru's techniques. Maybe he discovers some of Orochimaru's old research and figures out how to use snake summoning for his own good. Another reason might be that he is trying to find a new power source to defeat a very powerful enemy that normal jutsu can't handle.