Perhaps he discovers a new power source or material in his adventures that can be used to make bombs. And he realizes that bombs could be a useful addition to his arsenal to take on stronger foes. Also, he might be inspired by other characters who use bombs in the fanfiction world he exists in.
In fanfiction, Naruto could be making bombs to protect his village in a more extreme way. He might have seen the destruction caused by powerful enemies and thought that bombs could be a way to create a deterrent. Another possibility is that he is tricked or forced into making bombs by an evil force. But as he makes them, he could find a way to turn the situation around and use the bombs for good instead.
It might also be due to a sense of hopelessness. If he's faced with an enemy that seems truly unbeatable and he fears the destruction of the village or his friends, he could ask to be killed to prevent further harm. For example, if there's a powerful villain who has Naruto's friends hostage and the only way to save them is by Naruto sacrificing himself.
It could be that Sakura betrays Naruto's trust. For example, if Naruto entrusts Sakura with a very important secret, like something related to his family or his new power he's developing, and Sakura reveals it to their enemies or uses it against Naruto in some way, that would be a huge blow to Naruto and could lead to him crying.
One possible reason could be that Naruto feels he has become a threat to the village or his friends in some way. Maybe he has some sort of dark power within him that he can't control and he believes Sasuke is the only one strong enough to stop him. Another reason might be related to a prophecy or a greater threat that Naruto thinks his death can prevent, and he trusts Sasuke to carry out this difficult task.
One possible reason could be for a mission. Maybe the mission requires infiltrating a female - only place or getting close to a female - only group, so Naruto decides to turn himself female to better complete the task.
Naruto might be longing for a motherly figure in his life. He has always been alone and has seen how others have mothers. Tsunade, being a strong and caring woman in the village, could be someone he looks up to and wishes to have as a mother.
Maybe Naruto discovers some dark secret about Tsunade's past that she has been trying to hide. For example, something related to her lost loved ones. Naruto, in his usual straightforward way, confronts Tsunade about it without realizing how much it would hurt her, and thus makes her cry.
One possible reason could be extreme guilt. Maybe Naruto has done something he believes is unforgivable, like accidentally causing the death of a beloved friend while in a fight. He might think that death is the only way to atone for his mistake.
One reason could be simple curiosity. Naruto might be interested in seeing how fans from the Bleach fandom imagine different storylines. Another reason could be to gain inspiration for his own ninja world. Since Bleach has unique concepts like Soul Reapers and Hollows, Naruto could be looking for fresh ideas that he can adapt to his own battles and relationships in Konoha.
Perhaps the fanfiction alters the relationship between Kakashi and Naruto in a way that Kakashi doesn't like. Maybe it shows Kakashi as being overly harsh or unfair to Naruto in a situation where in the original series he wouldn't be. It could also be that the fanfiction contains plot elements that Kakashi, as a character, would find illogical or against the spirit of the Naruto world.
One reason could be to explore a different side of Naruto. Maybe the writer wants to show how even a character as positive as Naruto could break under extreme circumstances. Another reason might be to create a more complex and less idealized version of Naruto.