In this fanfic, a possible cause for Naruto's heart attack could be a hidden genetic defect. Maybe it was passed down from his parents or some sort of unknown bloodline trait that suddenly manifested. It could also be that he was exposed to a strange and powerful chakra that his body couldn't handle and it affected his heart negatively, causing it to malfunction and leading to a heart attack.
One possible cause could be over - exertion during a fight. Naruto is known for pushing himself to the limit. If he has been in a long and intense battle, it might put too much strain on his heart. Another cause could be some sort of poison or dark jutsu that specifically targets his heart function.
One possible cause could be a high - cholesterol diet. If a man eats a lot of fatty and fried foods over time, it can lead to plaque buildup in the arteries, which may trigger a heart attack.
Maybe he was poisoned during a battle. The poison could be slowly affecting his body and causing a fever as his body tries to fight it off. In fanfiction, the poison could be something unique created by an enemy ninja with special properties.
In the fanfic, it might be due to a ninja technique gone wrong. For example, if he was trying a new jutsu that affected his internal chakra flow and it ended up giving him a tummy ache. This could be a really interesting plot point as it shows the risks of experimenting with new powers.
Well, it could be that he ate some bad ninja food. Maybe he ate a spoiled ration from a mission. You know, in the world of Boruto, they might not always have the freshest supplies on long missions.
One possible trigger could be the memories of his past traumas, like the death of his parents or the loneliness he felt as a child. Another might be the pressure he feels to protect the village and his friends. Also, facing a very powerful enemy that he's not sure he can defeat could send him into a panic.
Maybe the horns are a sign of a new power awakening within Naruto. He could initially be afraid of them and hide them, but as he discovers their potential, he learns to embrace this new aspect of himself and use it to protect the village in ways he couldn't before.
It could also be that he ate some magical food that didn't agree with him. For example, he might have tried a new kind of magical sweet from Honeydukes that had a strange side effect on his digestive system. Another reason could be that he was under a minor curse. Some of the dark wizards or witches in the story might have cast a spell on him that caused his stomach to ache as a form of petty revenge or to distract him from something important.
One possible cause could be an allergic reaction. Maybe she's exposed to a new type of pollen or a strange animal in the story. For example, if she visits a new place with rare plants, the pollen might trigger her breathing problems.
In fanfiction, it might be a result of a curse or some dark magic. For example, a villain could have cast a spell on him that weakened his heart both physically and emotionally.