Maybe he was in the wrong part of town. The Socs consider certain areas as their territory, and if Ponyboy wandered into one of those areas accidentally, they would jump him to send a message. Also, it could be that he was with a Soc girl or was seen talking to one. The Soc boys wouldn't like that and would jump him to teach him a lesson.
One possible reason could be that he's a Greaser. The Socs might see him as an easy target just because of the group he belongs to. They might want to show their dominance over the Greasers.
In the fanfiction, Ponyboy could be getting jumped because he was mouthing off to some Socs earlier. He might have said something that offended them, and they decided to get back at him. Another reason could be that he was trying to protect one of his Greaser friends from the Socs, and they targeted him as a result.
In this fanfiction, it might be due to his poor living conditions. He doesn't always have the best food or a warm, clean place to stay. His body could be weakened from not getting enough nutrition, which makes him more likely to get sick. Also, the emotional turmoil he goes through with losing friends and dealing with the Socs could take a toll on his immune system.
One way is through detailed descriptions. For example, describe the look in the eyes of the attackers, how their muscles tense before they make a move. Another way is by using short, choppy sentences to mimic the quickening of Ponyboy's heartbeat. Also, having the surroundings play a role, like a dark alley with no way out can really build the tension.
One possible plotline could be that Ponyboy and Sodapop get caught in a sudden storm while out in the open. They're both scared as lightning strikes close by. They huddle together in an abandoned building, sharing their fears and memories from their tough lives. This situation could strengthen their bond and lead to some deep conversations about their dreams and hopes for the future.
One reason could be his own clumsiness. Maybe Peter is always in a rush and not paying enough attention to his surroundings. For example, he could be running down the stairs while looking at his phone and trip, resulting in an injury.
One reason could be that he was exposed to some kind of harmful chakra or energy in the village. Maybe some leftover dark jutsu residue affected his weak baby body.
In fanfiction, one possible reason could be that he was exposed to some kind of harmful chakra. Maybe an enemy sent a weak but insidious chakra wave towards him when he was just an infant, which gradually made him sick. Another reason might be related to the poor living conditions he was in. Since he was alone as an infant without proper care, he could have caught a disease easily from the dirty environment around him.
One plot idea could be that Sodapop and Ponyboy go on a road trip together. They face various challenges on the road, like car troubles and meeting some interesting characters. This would allow for a lot of character development and some really sweet moments between them as they rely on each other.
It could be due to a mental breakdown. For example, if he had been under a great deal of stress for a long time, perhaps from work or family pressures, and suddenly couldn't take it anymore. Or he might have some undiagnosed mental illness that led him to this extreme action. Also, a sudden and tragic event in his life like the loss of a loved one could be a reason.
One reason could be false accusations. Maybe someone frames Jaune for a serious offense he didn't commit, like cheating in an important exam or stealing valuable items from the school. Another reason might be his inability to keep up with the academic or combat requirements. If he fails too many classes or consistently underperforms in combat training, the school might decide to expel him.
Perhaps the fanfiction is following a specific plot arc that doesn't naturally involve Stiles. Like if it's a story about a new villain in town and the existing characters are dealing with it in a way that doesn't require Stiles' skills or presence. Also, some writers might be influenced by popular trends in the fandom that don't feature Stiles, and they go along with that to attract more readers.