Probably because your not supposed to wash them.
If your uggs just got wet from the rain or something,
spray them with a boot Protectant and a spray shield,
spray them until their damp and then brush out in one direction only with a sponge or cloth.
That should help,
but you really shouldn`t wash your uggs,
it will only wreck them.
Try getting some boot stands to put inside your uggs to hold them up straight when your not wearing them so the tops don`t fall onto the bottom because of the weight,
it will stop crease from happening too.
Hope I helped :)