When a rabbit lies flat, it makes itself look smaller and less threatening to the other rabbit. This can help to defuse a potentially aggressive situation. In addition, lying flat exposes the rabbit's vulnerable belly, which is a sign of submission.
Rabbits may also lie flat when they are feeling scared or anxious. This can happen if they are in a new environment or if they are feeling threatened by another animal. By lying flat, the rabbit tries to make itself less noticeable and less likely to be attacked.
If your rabbit is lying flat when another rabbit approaches, it is important to observe the body language of both animals. If the other rabbit is also displaying submissive behaviors, then the situation is likely to be peaceful. However, if the other rabbit is showing signs of aggression, then it is important to intervene to protect your rabbit.