From what you described it sounds like no virus was involved there, but I can't guarantee that if you want to know for sure autopsy would explain that.
She was still very young, baby rabbits live on mother's milk until they are 6-8 weeks old, from 4-5 week they start eating hay and high protein, calcium and phosphorus junior pellets, as much as they can eat, that is in addition to very rich mother's milk they get. As they grow they eat more solid food and getting stronger and then mother stops nursing normally around 6-8 week but some mothers can feed them until 3 months or even later.
So after 8 weeks babies should have unlimited junior pellets up to 250 gram a day, unlimited grass hay and unlimited water until they are 4-5 months old, sometimes until they are 6 months old, then you should reduce their pellets gradually to 50 gram a day, but you cut them gradually, just if you fed 4 times a day 50 gram you reduce amount and reduce feeding times to 3 and then 2 times a day and an adult can have once a day 50 gram. But in my experience with larger breeds babies are growing slower so they need richer diet up to 8 months sometimes, depending on circumstances.
Hay only diet from 8 weeks when they need to grow sounds really wrong to me tbh.