There is a very high risk of a rabbit dying from being alone since they fall into depression when they're not around another rabbit.
I'm not sure what the actual cause is. But I've seen rabbits die soon after they were separated and left alone in a cage.
I never knew what they died from because they weren't my rabbits but I think it might be that when they're depressed they decide not to eat and starve to death.
I've heard other pets do that as well such as dogs and cats when their owner dies or the owner has to find the dog or cat a new home.
The dog or cat falls into depression and won't eat and eventually dies of starvation. That's what I think happens to the rabbits. But I can't say 100 percent if that is the cause of the death of the rabbit since I've never owned any rabbits.
Shelters do actually recommend keeping rabbits in pairs though since there's less a risk of the rabbit dying when they get lonely.