Why does it seem like my cats always love to wait until I am walking passed them, and then run in front of me?

They will literally sit and wait until I am in just the right position before hurling themselves forward! I swear, every day I almost trip and fall trying to avoid my three nut cases. Sometimes I can't jump out of the way fast enough and they end up slamming into me or tripping me. Do you think they do it on purpose and laugh at me behind my back?
