Responses
Kiku goes back to the past after he realized that he was resembling his old lover(Shirou's granpa) with Kiku's(granpa's name) great son, it happened when they were looking into some photo album thus he started to remember everything. Although he didnt think imediately to go back to the past, my hint it was after his chat with Shirou's uncle. You can fully understand when they were doing sex(Kiku and Shirou) that he's feeling guilty staying there bc the burden of already knowing that you love someone else is tearing him apart.

Yeah, it has a bad ending(?)..
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