One possible reason could be a misunderstanding. Maybe Erza misinterprets Natsu's actions as being reckless and endangering the guild, so she takes harsh measures to 'teach him a lesson'.
Another reason might be that Erza is under extreme stress. The guild could be facing a great danger, and Erza, in her attempt to protect everyone, might accidentally hurt Natsu. For instance, if she is in a battle situation and Natsu gets in the way while she's using a large - scale attack, she might end up harming him without fully realizing it at first.
One possible reason could be for a practical joke. Lucy might be in a playful mood and think it would be funny to tie Natsu up and see his reaction. Another reason could be for his own safety. Maybe Natsu was about to rush into a dangerous situation without thinking, and Lucy tied him up to stop him. It could also be part of a training exercise. If Natsu needs to learn how to break free from restraints in combat situations, Lucy could be helping him by tying him up and making him figure out how to escape.
One possible reason could be a misunderstanding within the guild. Maybe some false information was spread about them, leading others in Fairy Tail to turn against them. Another reason might be a power struggle. If Natsu and Erza were becoming too powerful, some jealous members could have conspired to make it seem like they were betraying the guild. Or perhaps an outside force manipulated the situation to drive a wedge between them and the rest of Fairy Tail.
Perhaps she's under the influence of a powerful genjutsu. Genjutsu can distort one's perception and make them do things they wouldn't normally do. In this case, Kushina might be seeing Naruto as an enemy in the genjutsu and thus hurt him. Or it could be a test in a very extreme way. She might think that by putting Naruto through pain, she can make him stronger, although this is a very cruel way of thinking.
Well, without more context, 'her' could be any number of characters. It might be a new character created for the fanfiction. Natsu might want her to join Fairy Tail because he sees potential in her magic abilities. Fairy Tail is all about accepting people with different powers and making them part of the family.
One possible reason could be a conflict with other guild members. Maybe there's a misunderstanding or a fight that makes Natsu feel he no longer belongs. Another reason might be that he has a personal mission that goes against the guild's rules or values in the fanfiction. He could also be framed for something he didn't do and, feeling betrayed, decides to drop out.
One possible reason could be a misunderstanding within the guild. Maybe someone spreads false rumors about Natsu, leading others to believe he's done something wrong. Another reason might be a power struggle. If there are those in Fairy Tail who envy Natsu's strength and popularity, they could plot against him to get him out of the way.
One possible reason could be to seek new adventures. They might feel they've reached a point in Fairy Tail where they've done all they can and want to explore new magic and challenges elsewhere. For example, they could have heard of a far - off land with a different type of magic that they are eager to learn about.
Perhaps Natsu feels betrayed by the guild first. There could be a situation where he believes the guild has turned its back on one of his closest friends or on a principle he holds dear. This sense of being let down by the guild might lead him to take extreme actions that seem like betrayal. It could also be related to a power struggle. If there's a new power within the guild that Natsu sees as a threat to the guild's original values, he might try to oppose it in a way that looks like betrayal from the outside.
One great aspect of Natsu and Erza fanfiction is the exploration of their unique friendship. In many stories, their different personalities - Natsu's brashness and Erza's strictness - create interesting dynamics. For example, there are fictions where they team up for a mission and Natsu's unorthodox methods initially clash with Erza's by - the - book approach, but eventually they learn to work together better, leading to a stronger bond between them.
One reason could be that the fanfictions don't match his view of the Fairy Tail world. He has his own understanding of the guild, his friends, and their adventures, and when fanfictions deviate from that, he doesn't like it.