Why did O'Connor do it? She probably just wanted to show off and achieve notoriety, using a leftist-approved meme, i.e. bashing the Church. College students are always so brave and courageous while attacking the traditional culture; they will get pats on the head from their left-leaning teachers and kudos from their colleagues. However, as former Communist David Horowitz has pointed out, "the issue is always the revolution," no matter what the issue pretends to be.
The left is determined to root out religion from the culture so it can be replaced with something else, i.e. an almost religious belief in the state. Such incidents, like the piss-Christ photograph and the elephant dung-Mary, are designed to be a kind of cultural acid, slowly weakening the mortar that holds the bricks of western civilization together. Over time we become less sensitive to such displays, weakening our will to resist the cultural and political changes the left has planned for us.
As for Katherine O'Connor, she has her own cross to bare. If she gets off, it will be a close shave.