Anyone who's seen Anohana: The Flower We Saw That Day knows it's a tearjerker. What fans might not know, at least right away, is exactly what happened to Menma. While the anime revolves around the effects her death had on her group of friends and how it continues to affect them into adolescence, the actual cause of her death is only ever alluded to for a majority of the 11-episode series.
The subtlety with which Menma's cause of death is given helps amplify the inherent tragedy of what was ultimately an accident. Jintan and the rest of Menma's friend group spend years grappling with her loss, having felt that if they weren't wrapped up in their own emotions of the day, they would somehow have been able to prevent her death. With the way Anohana explores the human need to find responsibility and place blame in the face of tragedy, the mystery of Menma's accident is filled in through the examination of everyone's grief.
Anohana weaves together a story of five friends' feelings of anguish, revealing the cause of all the trauma with every step forward in the grieving process. As the anime progresses, it's periodically hinted that Menma's death involved the river. Jintan quickly stops Anaru from slipping into the water in Episode 4, and he becomes anxious when Menma nears the river to look at a carp in Episode 9, as if she'll fall in. However, the series finale reveals that the cause of Menma's death was her slipping and falling into the rushing river -- when there was no one around to pull her out.
By the final episode, everyone confesses their individual feelings of guilt, filling in the blanks of what happened that day when they were children. Menma had ran off to find Jintan, who had broken off from the group in embarrassment after Yukiatsu and Anaru tried to make him confess his feelings for Menma. Yukiatsu took this chance to tell Menma his feelings for her, only making for an even more overwhelming situation between the children. Menma knew that Jintan had feelings for her, having been told by Tsuruko earlier that day, so her choice to run along the river and find him was inevitable. Poppo admits to having seen the incident happen and how he felt totally helpless in watching Menma float down the water.
Menma slipping was purely accidental, but because of the events leading up to it, every one of her friends felt exclusively responsible in some way or another. A childhood death, and an accidental one no less, is an incredibly traumatic experience. As children themselves there was little any of the friend group could do, but that doesn't alleviate their sense of responsibility.
With the emotional whirlwind in the moments leading up to Menma's accident, everyone's ability to process something so devastating was incredibly hindered, and their sadness and grief internalized. Anohana's focus on death through the grief it brings -- and the anime's decision to depict Menma's death almost entirely through her friends' memories -- poignantly emphasizes the frustration of facing events that are unfortunately outside of one's control.
As important as Menma's death is to the overall story of Anohana, the actual event is delicately approached to the point of being missed by some. Through this approach, the story never emphasizes the horror of seeing her death play out, instead showing how an unseen tragedy can have long-lasting effects.