I'm not sure of specific 'izuku snaps at aizawa fanfiction' titles off the top of my head. You could try searching on fanfiction platforms like Archive of Our Own. They usually have a wide variety of fanfics, and you can use those keywords in the search bar to find relevant stories.
One reason could be that Izuku has been pushed to his limit. Maybe Aizawa has been overly critical or strict in a way that Izuku can't take anymore. For example, if Aizawa constantly doubts Izuku's abilities despite his efforts, it could lead to Izuku snapping.
In fanfiction, Izuku snapping at Shinsou could be due to various reasons. Maybe there's a misunderstanding between them, like a competition for a certain opportunity at school. Izuku, being under a lot of pressure, might lose his temper.
Well, 'izuku snaps' fanfiction could be about the character Izuku reaching a breaking point. It might explore a situation where he's pushed too far by the challenges he faces, like dealing with villains or the pressure from school. Maybe he snaps and does something completely out of character, or perhaps it's a moment of self - discovery where he realizes he has to change his approach to things.
One good one might be 'Aizawa's Guidance for Izuku' which focuses on their unique mentor - mentee relationship evolving in an interesting way. It has great character development.
Some dedicated My Hero Academia fanfiction forums might also have these stories. You can join these forums and ask around. Members are usually helpful in pointing you to the right stories.
I'm not sure specifically as I haven't read this exact fanfiction. But generally, in fanfiction, it might involve Izuku in a situation where he has some sort of intense encounter or reaction related to the 'scarecrow' character, perhaps in the context of a battle, a psychological struggle, or some other plot - driven scenario within the world of the original story it's based on.
In some fanfictions, Aizawa might be under the influence of a villain's quirk that makes him act against his will. So, he kidnaps Izuku, but later on, he realizes what he's done and tries to make amends. This could lead to a story full of drama and redemption.
It could be Aizawa himself if he's being controlled by something or has some sort of ulterior motive that is not in Izuku's best interest. But if not Aizawa, then there could be a hidden villain who is manipulating Aizawa to kidnap Izuku.
Perhaps Izuku was skipping class and went to the roof. Aizawa noticed his absence and suspected he might be there.
Another option is to ask in My Hero Academia - related forums or groups. There are many fans who are into fanfiction and they might be able to recommend some good 'izuku snaps' fanfictions that they've read. They could also give you tips on how to find more, like if there are any niche websites or specific authors known for this type of story.