Last Week My Brother Who Was 30 Years Old Passed

Last week My brother who was 30 years old passed away. About 3 years ago he was diagnosed with stage 4 non hodgkins lymphoma. He had been through mulitple chemo treatments and radiation. In October 2014 he had a bone marrow transplant. As of January 2015 he was cancer free. Around the end of February his health started to rapidly decline. He literally began to lose his mind. Nothing he said made sense, it became hard for him to walk, or do any normal routine thing. Eventually he barely speak and it took him a while to get out what he was trying to say. Around mis April he was admitted to the hospital. The doctors performed many tests and all of the results were negative. they believed the cancer had come back or that maybe something was attacking the brain but they found nothing. They thought he was starting to have seizures but those tests were also negative. In his last days he could not talk he could not walk or even sit up right. He had an oxygen mask on due to the fact that his oxygen levels were very low although he was breathing on his own and he had a feeding tube for nutrition. It basicaly seemed like he wasnt even there at all. They believe that a few days before he died he may have had a stroke but there were no tests done to confirm this. His eyes started to get a yellow color to them so they believed he had Jaundice. He was put in comfort care until he passed and we were told there was nothing else that could be done. Unfortunately his wife declined the offer for an autopsy and I have no idea why this happened. I was just wondering If this is common after a bone marrow transplant to just be cured and then randomly get sick and die with no answers? Im really confused and i Dont understand what happened to him.