Ask the vet - Rabbit
Why do rabbits thump with their hind legs?
Vet at FirstVet answers:
Thumping is a way for rabbits to warn each other of danger. They mostly thump when they are stressed or afraid.
Rabbits have strong muscles in their hind legs, but a relatively fragile skeleton. They have been known to break their backs from thumping so hard in response to extreme fright. This is however very rare.