My theory now based on the shoujo ai tag is that Sayoi was in love with Asami. Since they're both moving they probably were discussing the future; Asami may have brought up having a boyfriend (or Sayoi having a bf) in the future off-hand and makes Sayoi realize that her love/crush is unrequited, then thinks that since they're both moving anyway it's better to forget her love for Asami, and Sayoi ends up having an extreme self-defense reaction where her brain creates a Voice and deletes Asami from her memory entirely.
Just a theory of course. It'd be interesting if the shoujo ai tag is referring to smth else and isn't related to Sayoi and Asami at all.