When a buck falls over after mating with a rabbit doe, it is commonly referred to as a "mating flop" or "post-mating flop." It is a behavior commonly exhibited by male rabbits after successful mating.

During mating, the male rabbit may become momentarily unbalanced or lose its footing, causing it to fall to the side or flop over onto its side or back. This behavior is typically brief and temporary, and the buck will quickly regain its composure and resume normal activity.

The mating flop is believed to be a result of the intense physical exertion and excitement associated with mating. It is a normal behavior and does not indicate any health problem or distress. It is important to provide a safe and comfortable environment for rabbits during mating to ensure their well-being.