Sometimes, for one reason or another, we bunnies decide to just stop eating. Sometimes, a bunny gets scared and everything inside just stops working.
Nothing moves out so nothing moves in.
Sometimes, a bunny ingests too much fur and it doesn't pass properly. Everything gets stuck and shuts down. Sometimes, a bunny gets too many treats or not enough hay and gets gas. Everything stops working. There is a fantastic article which details everything you need to know about Stasis.