My view on it is that when you are younger (like primary/elementary school age) you most likely don't know what homosexuality is, so you get crushes on girls because that's the 'social norm' and it's all you know. Once you start to learn that it's possible to be homosexual, however, you might start to think that it's possible to have crushes on guys too, so you might start thinking of them that way. It's not so much that you suddenly decide to like guys, it's more to do with realising that you like guys once you know it's possible to feel that way to the same sex.