While eating, playing, or walking around, your bunny may suddenly roll or throw him or herself on one side and at times roll on to his or her back. This keeling is what is known as a rabbit or bunny flop.
At times, there might be a “buildup to the flop as a bunny turns his/her head a few times before finally rolling over completely onto his/her side” 
Rabbit flop gif
If you have never seen it before, the bunny flop gif below should give you an idea of what it is, to avoid being startled in case it just happens to your pet.
Why is my rabbit flopping?
If it happens, many new rabbit pet owner may assume something is wrong with their furry friend. They may think he is unwell or just fainted.
However, you do not have to worry since this body language means he or she is relaxed. Your furry friend is only trying to tell you that he or she is happy and relaxed, i.e., it may indicate that your pet is happy, contented, relaxed, and trusts you. Your rabbit also feels safe in his environment, something that any prey animal hardly does.
In verbal terms, he or she is trying to say, ‘I am very relaxed, and I may want to take a nap’, ‘I am very contented with my life’, or something close to that.
At times, a flop could mean otherwise, i.e., “as a mild insult with rabbits that are not good friends.” However, in most instances, this is hardly the case.
To correctly decipher this body language, you need to consider the context. For instance, if your bunny is happy and seems satisfied with everything, it means he is relaxed.