Bra-AS because the cup size arrive to that.
Anyway for why the plot changed, is very simple, MC was there, even if he didn't want to change the original plot his simple presence changes it, unlike other reincarnation stories where it happens because the MC have a main role and wants to ensure the best ending or ends up interfering because their presence is simply too big.
Most likely the demon would have originally slayed the guards or rampaged more in the city, but it noticed MC strenght or even if MC wasn't trying super hard it did enoguh to stop it, that made it run out from the city, MC tried to see if somethin gwould happen up to the last second, but it didn't, the girl didn't awaken some power, hero didn't come, nothing happened, and as MC was a hardcore fan he knew that event didn't exist in the original game, he had to get in the battle.
Now the only thing I really disliked of the chapter was how MC screamed the girl name like if it was a otaku fanboy, hate when characters do that crap and instill suspects in the people hearing.