Head and chest injuries can take time to develop. Thus, they get up and walk away. From what I heard, he developed cardiac tamponade as well as a bleed in his head.
Cardiac tamponade is when there is bleeding into the stiff sack that surrounds the heart. As the blood collects around the heart, it compresses it so that it cannot fill adequately and pump blood. If it develops due to trauma, it can come on rapidly or sometimes be a bit delayed. It is lethal if not diagnosed and treated rapidly.
The head bleed can be a few different things depending on where the bleeding is occurring. It can cause rapid deterioration in some cases or sometimes it can be delayed by several hours as the blood collects and compresses normal brain. In addition, with brain trauma, swelling develops leading to increased pressure and compression/disruption of vital brain functions. Surgery to open the bony part of the brain and allow the pressure to subside can sometimes be lifesaving.
So it is not that odd that he initially got up and walked away despite serious injuries. Some news reports said there may have also been a complication in his care. Perhaps they meant that the initial diagnoses were not made quick enough to treat through surgical intervention before the damage was already too far advanced.
Very sad story no matter the details. Seeing the types of stunts they do at the x games, I am surprised that this is the first death.