I think our obesity is a big part of it. Seems kind of obvious to me. Nobody questions that we have a higher rate of obesity or that obesity is commonly "managed" with expensive drugs addressing symptoms for years on end.
Another big part is that we go to extreme measures near the end of life. I once read that 25% of our spending is on people that are in the last year of their lives.
Also, I think malpractice lawsuits and insurance might be a big part. Don't know how big.
At the root of everything, I think the situation was doomed by 2 fundamental flaws: 1) insurance should only address catastrophic, unlikely emergencies, not routine healthcare, and 2) individuals should buy insurance, not employers. Those two things would have helped a great deal. Now, people would have had to get higher salaries all along since they're paying for insurance and I'm not against the government covering the routine healthcare that I think insurance should not address, but if things operated that way, at least market forces would have had a chance to put some pressure in the right places. Considering how stupidly we do things, I'm amazed it all works as well as it does.