Drinking too much water which causes a lack of energy. Being bred too many times without any rest between litters. All of the above. Very long fur. All of the above.
Rabbits just love to breathe stale air.
Circulated fresh air is very healthy for rabbits. Rabbits hate fresh air with a passion. It helps rabbits breed better and produce more litters. Sick rabbits and rabbits you have recently purchased. All the rabbits in your rabbitry.