Ahhh... I never read LB in english until now, hence why when I was reading Haruka, I was like "wait, where are the flashback scenes?!", forgotting those came from EX
That said, I'm pretty sure Kanata isn't the type of character who would try things for the hell of it, so a bit like Kyousuke who went hardcore in Rin2, I'm pretty sure she had a reasoning for her attitude, although it sort of backfired. I guess I should read again her route. Sure has been a long time.
That said, she is definitely not the one behind the flyers/graffiti, considering she went postal when Riki was about to see them. So an external factor, be it a commitee member or Kyousuke himself.