>>16350659
Recently it was heavily implied that Kasen was part of the group that proposed the Barrier- but she has been complaining about it and undermining it throughout the manga.
Yukari had always assumed that Kasen was 'on her side', and that Kasen chilling out in the mountains for years was tacit approval of Gensokyo as it is now.
It would appear that Yukari and Kasen helped set up the barrier for entirely different reasons, but we can only guess as to exactly what those reasons are.