There are a few reasons why your bunny might keep freezing. One reason could be that they're trying to conserve energy. Bunnies are prey animals, so they're always on the lookout for predators. If they think they're in danger, they'll try to stay as still as possible so they don't attract attention.
Another possibility is that your bunny is suffering from a medical condition called hypothermia. This happens when their body temperature drops below normal and can't warm itself back up. Hypothermia is especially dangerous for rabbits because their fur isn't very thick and doesn't provide much insulation. If your bunny is shivering or has cold, purple ears, it's time to take them to the vet right away.
Finally, some bunnies just tend to be more high-strung than others and get scared easily. If you think this might be the case with your bunny, try providing them with lots of hiding places and toys to help them feel secure. A calm, relaxed environment will go a long way in helping your bunny feel comfortable and safe.