Likely passed out in Las Vegas or fell down some stairs. That can kill people his age.
This is exactly what happened to my mother MULTIPLE times in the years leading up to her death five years ago.
My son and niece witnessed these mini strokes. My mom would start to shake and be out of it for about one minute. When she came out of these episodes she would have no memory of it.
She lived alone. Twice she fell. Once into the bathroom vanity. Another time getting out of her recliner. Each time she sustained several bruises. Plus, older people bruise very easily. The bruises also seem to last longer.
Eventually, mom had a massive stroke and was found on the garage floor by her neighbor. She died two days later in the hospital.
I suspect Biden had a stroke similar to this in Vegas. Fell down and bruised himself. He has a history of aneurysms