I am so very sorry this happened to your rabbit.
It seems there was bacteria festering in the wound that had healed closed. Some bacteria can thrive in both oxygenated and oxygen-free environments. There are also bacteria that can only thrive without oxygen. Those types of bacteria can fester inside a closed wound. An abscess forms in an effort by the immune defence to wall off the infection. Rabbits’ immunesystems are more about walling off infections than fighting them off. This is why rabbits are so prone to the development of abscesses. The abscess can involve bone and the bone gets destroyed in the process. It’s called osteomyelitis.
An abscess can grow very quickly, it can happen in hours. The prognosis for healing is very poor when there is bone involvement. It’s awful to have to have a bun pts, but sometimes it’s the only kind and loving thing you can do for your bun.
Here is some info about abscesses in rabbits: http://sawneeanimalclinic.com/downloads/abscesses_in_rabbits.pdf
Thank you for caring so well for your bun. I’m so sorry she didn’t make it.