Ok I read Nezchans take in the other thread - I guess that would make some sense but not totally. How did Alice know to look for Yukari, and why think she had done something? She thought Marisa returned her feelings from the start right? So why suspect something wierd?
I guess because it's Gensokyo. In a world of near constant incidents, it's natural to think that any sudden event is caused by someone or other messing with shit.
As to why Yukari, I guess because she kind of has that whole yokai mastermind shtick going on. Plus, if the confession happened on the some night as the party, then Alice might have thought of Yukari since she was kind of creeping on them earlier.
Those are my thoughts, anyways. I'm not sure I 100% buy that explanation myself.