So, in a desperate bid to try and rescue the happy ending she tried to give you, she attacked Sayori. Monika recognized that you, the Player, deleted her likely for ALL the other girls, not just Sayori. She will then force the game to close and delete the other character files, including herself.
If Monika’s character file is deleted (as explained in Monika’s Ending) before starting a new game in Act 1, Sayori appears to realize that she is trapped in a game. You might be interested: Quick Answer: Ancient Japanese Literature? What happens if you delete everyone but Monika?
