Why do rabbits freeze?

Today Rex has frozen twice, the first time was when he saw a couple of birds fly over the house (I assume this is what he was watching) and just sat there dead still for about 10 mins, and tonight when we had finished watching a film with low light, we let him out as he was protesting about being in his cage and he froze again on the mat.

Maybe for protection as in "if I don't move they won't see me and therefore I won't be their dinner". He may have thought the birds were eagles or something and later on maybe he heard something or saw something. Rabbits are on everything's favourite menu (including some humans!) so even though he's a pet, instinctively he will protect himself.

As a defence mechanism because they're a prey animal.

Rabbits are prey animals.
Sudden sounds,smells and sights will make them freeze so they wont be noticed by any predators.Its a natural reaction to them.