In general, when something like this happens...a seemingly healthy person dies unexpectedly...it is usually because there is a congenital defect that is undiagnosed. Not to blame the parents or the child's physisicians...but how many of you have a full body CT Scan or MRI just to "check" if you have any vessel malformations or aneuryms somewhere? No one. You don't have that kind of testing unless there is a reason, but, in a situation like this, riding a coaster or other type of ride where there are pressure changes, if you do have something like that, the possibility for rupture is there. My guess is that they will find something along those lines in the autopsy.
Not that any of that makes this any less tragic. My heart goes out ot the family of this boy.