Maybe he was poisoned. There could be someone who slipped a harmful potion ingredient into his cauldron without him noticing. It's a common plot device in fanfics to create drama.
Perhaps there was some sort of magical interference. Another student might have accidentally (or on purpose) cast a spell near his cauldron that disrupted the potion - making process. This disruption could have released some sort of negative magical energy that affected Harry, causing him to collapse. It could also be related to a pre - existing condition that was exacerbated by the fumes or the magical properties of the potions ingredients he was using.
It could be Malfoy. He often has a vendetta against Harry and might have found a way to make him collapse during potions class, either by tampering with his ingredients or distracting him in a way that leads to a mistake in the potion - making process.
Maybe Snape wants to train Harry better. By using potions to change Harry's abilities or characteristics, he can expose Harry to different challenges and make him a stronger wizard. For instance, if he gives Harry a potion that temporarily takes away his ability to use magic, Harry will have to learn to rely on other skills.
One reason could be that he's facing extreme pressure in potions class. Maybe he's constantly compared to others, like Snape always having high expectations of him. Another reason might be that he has some kind of emotional turmoil related to his past traumas while in the potions environment. For example, the smell of a certain potion might remind him of a difficult time at the Dursleys.
In a lot of these fanfictions, it could be that Harry has a secret mentor. This could be an old wizard who has been secretly guiding Harry in potions, away from the prying eyes of the Hogwarts staff. This mentor could have seen Harry's potential early on and decided to help him develop his skills in private. Another aspect could be that Harry's own determination and his need to prove himself in a different area other than Quidditch or fighting Voldemort drives him to secretly practice and become good at potions. Also, the idea that there are hidden aspects of magic within him, perhaps related to the founders of Hogwarts, that only manifest in his potions - making ability.
It could be that he has a bad accident in the potions lab. For example, a potion explodes and nearly kills him or one of his friends. This traumatic experience makes him lose his confidence and interest in potions, leading him to quit as seen in some fanfictions.
One possible reason could be overexertion. Maybe he was using too much magic outside of class and it drained him. Another reason might be some sort of magical illness or curse that he's been secretly dealing with.
Severus Snape is another character who could be considered a potions genius in fanfic. In the official story, he was a master of potions. Fanfic writers might explore more of his early days as a potions prodigy or create scenarios where he imparts his deep knowledge of potions to other characters in unique ways.
One common theme is Harry's exploration of potions. He might be on a quest to find the rarest ingredients or perfect a difficult potion.
Yes, there are quite a few. You can check out fanfiction websites like Archive of Our Own. Just search for 'Harry likes potions' and you'll find a variety of stories with different plotlines and writing styles.
In some fanfics, Snape may use a potion similar to the Felix Felicis, the luck potion. But instead of just bringing luck, this modified version could be used to change Harry's fate or the course of his actions. It could make Harry more inclined to make certain choices that Snape desires. Also, there could be a potion that can temporarily suppress Harry's magic, which Snape might use to keep Harry out of trouble or to protect him in a different way.