I don't like her but I think she hit him because she thought he was hurting her nephew. Deep down she must feel at least some responsibility otherwise she wouldn't have taken him in.
but i thought it was yanagi who asked why she's here?
Remember his little brother told him that she usually goes in the afternoon/night. And it’s still pretty early in the day so it’s unusual that she’s here when he is normally there. He tries to avoid coming at the same time because he doesn’t want to cause problems.
ugh the nerve of that woman to smack Asanagi-sensei out cold like that and asking Yanagi why he's there. can't a guy visit his own parents' grave? and actually Yanagi already did her a lot of favor by timing all his previous grave visits just so he wouldn't run into his aunt who obviously did not want him around while he was growing up