Perhaps in the fanfiction, Aizawa gets fired because of a new strict educational policy that doesn't align with his unorthodox teaching style. He then has to find a new way to support himself while still being involved in the hero community in an unofficial capacity. He might start mentoring some of the students secretly, and as they succeed in their hero activities, it draws attention to the fact that his teaching methods were actually beneficial, leading to his possible re - employment.
One possible plot could be that due to some false accusations of improper teaching methods, Aizawa gets fired. His students then band together to prove his innocence. They collect evidence of his effective teaching, like how he has helped them improve their quirks and grow as heroes in training. Eventually, they manage to clear his name and he is reinstated.
It could be a new administrator at the school who has a very different vision of how hero training should be conducted. This person might see Aizawa's methods as too old - fashioned or too lax, even though they are actually effective. So, they push for his dismissal.
Perhaps in the fanfiction, Aizawa gets One For All and decides to keep it a secret. He then starts training in isolation to master it. As he trains, he discovers new aspects of the power that are different from what previous users experienced, due to his own unique quirk and personality. He might use it to secretly protect his students from threats that are beyond the scope of his normal abilities.
Well, it could be that Harry discovers some major misdeeds by Fudge. Maybe Fudge has been covering up dark magic activities within the Ministry. Harry then finds evidence and presents it to the proper authorities, leading to Fudge's dismissal.
Well, it could be that Lloyd made a major mistake in his strategic planning for the Geass-related operations. Maybe he miscalculated something crucial about the enemy's movements or capabilities, which led to a significant setback for his side. And as a result, his superiors decided to fire him.
Well, in a fanfiction, SpongeBob could be fired due to a misunderstanding. For example, Mr. Krabs misinterprets something SpongeBob says as a threat to quit. SpongeBob is devastated. He then tries different jobs like being a delivery boy for a seaweed company. However, he misses the Krusty Krab so much. In the end, Mr. Krabs figures out the truth and welcomes SpongeBob back with open arms.
In such a fanfic, perhaps Harry discovers some long - hidden, damning secret about Dumbledore. Maybe it's related to Dumbledore's past actions with Grindelwald. Harry, being the honest and justice - seeking person he is, feels compelled to expose this secret, which then leads to Dumbledore being fired from his position at Hogwarts.
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.
One possible reason could be his performance at work. Maybe he repeatedly failed to meet his targets or made too many mistakes. For example, if he was in a sales job and couldn't close enough deals, his boss might decide to let him go.
Well, one possible plot could be that Squidward gets fired because he finally snaps at Mr. Krabs over his cheapskate ways. He then has to figure out how to pay his bills and find a new job. Maybe he tries to start his own art business but struggles with marketing it.
He might lose his control over his own quirk for a while. Since his quirk is erasing others' quirks, if he's hit by a strange quirk, it could disrupt his own quirk's function.