One of the most common reasons rabbits run around fast is because they are feeling joyful and playful.
Rabbits have a lot of energy to burn, especially in the early morning and evening when they are most active (crepuscular).
If you have multiple rabbits, they may engage in fast chasing and racing as part of their social interactions.
In some situations, rabbits may run around quickly as an escape response to something that startled or scared them. It's their way of fleeing from a perceived threat or danger.
Rabbits communicate through body language, and running around can be a way for them to express their feelings.