The fact is, we just don't know. But it is certainly possible to rupture, tear, or dissect a vessel without any pre-existing, visible bulging aneurysm. The defect can be microscopic. Imaging is very, very difficult in horses for so many reasons, but it would be an extremely fruitful place to start, IMO.
That is solidly on the list of possibilities, although I'm told this is seen much less commonly in the days of modern dewormers. The necropsy findings, I'm also told, are very distinct between verminous arteritis and cystic medial necrosis. (which is the histopathological common denominator in most aneurysm/rupture cases I've read about in the available literature)
I think the medical / fitness research angle is very important, and I don't find Kingpin's death in some way 'ok', but if he died doing a trot set the week before Rolex of the same cause, would we be talking about banning trot sets?
Actually, I don't think so- we'd be saying how sad it is, or something like that. But- it would not involve a jump, going at speed, in front of thousands (many horse lovers, but not many that fully understand some of what we are talking about on these threads...) risking a human life.
I realize that sounds cruel, but I do think it would be different. IMO, as sad, but from a different perspective.