cause of death? please help.

So iv owned a rabbit for about 2 years, I got it a companion about 3 months ago. They bonded together and acted as best friends. So last night the new one escaped out of my room as I was getting there veggies, and I heard sqeeking, literally. I thought the foster dog had a toy so when I came out I found poor little nombnomb in the corner, with the beagle on it. I pulled the rabbit away and got it in my room. I checked out for any marks. I found none. I comforted him, and puthim back with hazel. She nestled up with him and groomed him. I never seen him for scared. Hes always happy running around with his ears up. But he just sat there in a corner. I puttogethrr a room with blankets and veggies. He was isolated for a shprt time and was on his side breathing normally. My mom made me pick him up and comfort him, I knew it was a mistake but I did, he started having seizures in my arms and within a couple minutes he died. I balled my eyes out when he was having seizures. I just want to know what caused his death. Please help.

Are you sure there were no bite marks? It's sometimes hard getting through the fur to see their skin well. If you're sure he wasn't bitten then he may have had heart failure. Rabbits have been know to die from fright or be injured from freaking out. One of our rabbits got out of her cage (outside) and our dog decided to have fun with her. She wasn't bitten but I could tell they had mouth'd her because some of her hair was wet. I found her hiding behind a board and very scared. We were so afraid she would die from the trauma, but she recovered.

Either trauma or internal injuries