The only logical explanation is that this is a fanfic prequel to Sekai de Ichiban Oppai ga Suki. "But J, this girl is Rika, not Chiaki", you say. Buckle up, we can explain that.
In middle school, Chiaki had an accident. Ever seen KimiNozu? That kind of accident. Here though, she loses all memory and wakes up to become Chiaki. BUT! She is left with one thing only, her obsession with oppai. The rest is history bois.