I’m very sorry your bun died. It’s very difficult to guess what caused his death. I very much doubt it had to do with the fight he had with his brother.

Rabbits sometimes seize when they die. It doesn’t give any good clues about any particular cause of death. The rabbit disease RVHD can cause seizures, but many other things can too, for example e cuniculi or acute heart failure or a vascular stroke (which is rare but does occur).

Does your other rabbit seem healthy? Its good if you can give him extra attention now when he has lost his brother.

I’m sorry for your loss. I agree with Bam, it’s difficult to know what happened. Since he is so young though, it would suggest there was something wrong with him. Perhaps he had a congenital defect of some sort.

So sorry for your loss. It’s so hard to know what happened in cases like this, and I know that only makes it rougher, especially since it was sudden and he was still so young. As others have said he probably had some existing health issue that was hiding, try not to play the blame yourself game. It is good you were able to be there with him his last moments, I am sure he appreciated it.

Only thing I would recommend is if you can maybe getting his brother a good health check to make sure it was nothing genetic that could also strike him, also this would rule out anything contagious. That and give him some extra love which I am sure you already are.

Again sorry.