I think it was the 'shrooms and booze and -- do they have drugs on Spira?

Theory:
After Tidus disappears, Yuna gets all depressed. Then Rikku and Wakka try to cheer her up. It doesn't work. She goes to that lake (probably has an official name, but for our purposes "Make-out Lake), cries for a while, and drinks liquer. That doesn't cheer her up, so she tries the local flora. That changes her personality to the point where she thinks it's cool to dress like a slut.

Theory 2:
It was Square-Enix. For the bling-bling.