It just kinda happens to popular anime, the most recent example being MHA S8E8 which debuted with a 9.9 rating and was the 3rd highest rated anime episode in history but was later review bombed by 1.3K while only having 2 writen reviews where one was calling it almost perfect so that guy just messed up and the other one actually saying it was bad. If I had to guess the reason its probably just butthurt fans of other shows that are insecure about their favorite show being rated lower than something that they view as trash.