What the fuck does that to a man?
An Embolism?
Quite possible, or like was said before, there are a number of respiratory infections that can present with that side effect, coughing up blood, but with a pulmonary embolism it would be a lot of blood and with a respiratory infection it would mostly be bloody sputum. I'm gonna guess pulmonary embolism though, given how quick he went from "nah I'm pretty alright, this cough is annoying though" to "fuck I think I'm dying/about to lose consciousness". The wildcard is the sinus symptoms he was having before that, but that could be totally unrelated, maybe he always had sinus issues and it was just a normal thing, and now everyone is microanalyzing everything that led up to it.
I'm inclined to think if it was a respiratory virus/pneumonia, he either had 'walking pneumonia' [usually found in younger people, it just slowly accumulates fluid in the lungs and will eventually knock you on your ass or present enough symptoms that you wind up in urgent care and get a script for antibiotics before it gets really, really bad, like ICU level bad] or something like that, but given that he was still going to events, still doing work-related stuff, still active with his family and most notably, driving a fucking racecar, I'm inclined to think it was a pulmonary embolism. It could be that the nagging cough was just incidental [had a chest cold or allergies or some shit] or it put enough stress on his body to cause other shit to stop working right, thus PE.
I wonder if they're going to do an autopsy. Usually with younger people who are otherwise healthy, they'll perform one, but there's no guarantee [with an illness like that, it's usually left up to the family unless there are suspicious/unusual circumstances] of one being performed and even if they do one, there's no surety that the results will be released to the public. I'm pretty damn curious but in the long run, I guess it doesn't really matter - it was definitely respiratory. The family may not want an autopsy to avoid altering his body, if he was an organ donor they may have kept him on life support long enough to extract useable organs, which would have been substantial since he was a pretty healthy, younger guy who was in pretty decent shape. Not to PL, but I know when I had a relative die suddenly and of rather strange circumstances, at first we were told an autopsy would be performed - then later on the coroner called and said 'nah we're not doing one, unless you want to pay like $1800 for it', and we kinda got together and decided that although we were curious about the cause of death, it wouldn't really bring any closure - said relative is still dead, knowing what killed them isn't going to accomplish a whole lot and it isn't going to bring them back. $1800 is a good chunk of change when you already know you're going to be paying for all of the other fucking expenses involved when someone dies. Cremation costs, or astronomically more if you're going with a burial, plot, headstone, casket, vault, all of that shit on top of an actual funeral. Not that finances would really be a constraint for the Busch family, I'm sure the funeral is going to be fucking massive.
Wild that before, they were probably planning around the race, maybe thinking about Memorial Day weekend plans, probably still birthday cake from his son's birthday in the refrigerator - and now a day later, they're planning a fucking funeral and trying to figure out all of the estate bullshit. Life comes at you fast. I'd hope Kyle had a rock-solid will and most of his estate shit planned considering his profession, though, so at least there's that. These guys know that any given Sunday, they can go from a racecar driver to a mass of charred up unrecognizable meat, if a bad crash happens.