There could be many reasons behind this, it would be best to ask a vet, but my opinions are that:
- The rabbit was possibly quite young/this was her first litter, and she wasn't ready for it, which caused her stress.
- She could have become too stressed or scared/spooked by something, which scared her to death.
- Complications with the kittens, they could have been too big or too hard to get out.
- Its possible that she simply bled to death, or was not strong enough after/during the labour to survive.
