One possible reason could be a dark magic influence on James Potter in the fanfiction. Maybe he's been cursed or possessed, causing him to act out of character and disown Harry.
Some fans might create it as a form of shock value. They want to stand out by presenting a very different and controversial take on the relationship between James Potter and Harry.
One reason could be for the sake of novelty. Fanfic writers like to create unique and unexpected scenarios, and this is a very different take on the Potter family. It grabs readers' attention.
The threat to his friends. James is very loyal to his friends, like Sirius, Remus, and Peter. If they were in danger or if something bad happened to them, it would be understandable for him to cry. For example, if one of them got seriously injured or was in a life - threatening situation, his emotions would be on full display.
It might be related to power dynamics. If in the fanfiction, James has a lot of power and misuses it in a way that affects Harry or those around him, Harry would hate him. For example, if James uses his influence to push through unfair rules or to marginalize certain groups that Harry supports. Also, if the fanfiction changes James's personality to be more self - centered and less empathetic, Harry would likely dislike him.
One reason could be jealousy. Maybe Albus has some special ability or is getting more attention from their parents. Another could be that James is just following in the footsteps of the old school bullies he might have known, thinking it's normal behavior. Or perhaps he sees Albus as too different from what he expects a Potter to be.
Perhaps James had some hidden magical ability that he was unaware of until that moment. It could be something that was passed down through his family but had never manifested before. This ability could have protected him from Voldemort's killing curse. Or, it's possible that there was a plot by some other characters to save James. There could have been a group of wizards or witches who had been secretly working to keep James alive because they knew his importance in the fight against Voldemort. They might have used some sort of time - turning magic or a protective charm that Voldemort couldn't break through.
The disownment could also change the relationships between other characters. For example, Harry's friends might become more protective of him, or some characters who were once neutral might side with Harry against James. It can also lead to a rift in the wizarding community if James is a well - respected figure. People might take sides, causing political and social unrest within the magical world. This can completely shift the focus of the story from the original plot to one centered around family drama and its consequences.
One possible reason could be that he was poisoned. In the wizarding world, there are many dark spells and potions that could cause such a reaction. Another reason might be extreme motion sickness if he was on a wild magical ride, like on a particularly unruly broomstick or a flying magical creature.
Well, it could mean in a fan - made story (fanfic), the character James disowns Harry. Maybe there's some sort of conflict or plot development where James, for whatever reason, decides to no longer consider Harry as part of his family or group.
In fanfiction, Harry could faint due to some sort of magical disease or curse. There could be a new, made - up illness that affects wizards and one of its symptoms is fainting. Or perhaps he's in a situation where he's cut off from his magic for a while and his body, which is so used to having magic, goes into shock and he faints. It could also be that he's in a place with really strange or strong magical energy that his body can't handle, and this causes him to pass out.