my rabbit used to do it all the time but she was a grumpy old thing, i think they do it to scare predators away with the loud sound because mine always did it when cats were around and at night whenever a fox came in the garden. she wasnt scared of cats tho, she loved nxt doors cat because shes the same colour as her mum and she used to bound up to her and sniff her on the nose and then get a wack. she also used to chase the cats out the garden and the squirrals but she was a very big rabbit so she scared them abit more than the smaller ones lol