I don't have a study, but I have myself. Ancedotes don't equal statistics but I can see how it happens.
For me:
As of January 1, 2011. Happy, healthy, planning a couple ski trips.
As of January 18, 2011. Happy, healthy. $3,000 in medical debt.
It wasn't a really hard road to get from 0 to $3,000 in 3 weeks either. I currently work for one of the top 20 largest companies in the US. I have a United Healthcare PPO (the largest carrier in the country) and Delta Dental (also the largest dental carrier). So I don't work for some fly by night startup with an off brand insurance, this is blue-chip benefits at the most stable kind of job you can have.
I had previously scheduled my wisdom teeth to be removed, had them removed no problem. They paid 80%, I paid 20% plus the deductible. Total cost to me was $450 out of pocket. No big deal and expected.
Not a week later, couldn't breath, thought it was either an asthma attack or allergic reaction to something I just ate at a restaurent. Go to the hospital, its not either. After a cat-scan, x-ray, a couple hours with a GI and another doc I find out it is something completely different and previously unknown. Controllable, not a big deal, but the symptoms were quite scary.
Get the bill, $2,100. My insurance declines $1,900 worth of charges since it was an "illness" and they don't cover that in the ER unless you must stay overnight because they don't want people using the ER as their primary care physician (understandable). So apply my $500 deductible, and I'm on the hook for basically the whole thing. Call it $2,000. (this is pending appeal). Mind you, if they approved it they would have paid 90% and I would have paid my $500 deductible, so I should have sandbagged it and stayed overnight apparently (while running up twice the bill).
C) Had to get some additional dental surgery. After the last experience expected to pay 20%, but for whatever reason they only cover 50% (since it was damage to my gums from retainers as a child, which apparently is not their problem) and I'm on the hook for an additional $925.
Now, writing checks for $3,000 won't bankrupt me by any stretch. (mind you, I'm actually writing more like $4,000 but a bunch of it is fair and expected, the $3,000 is the amount above and beyond what I think I'm supposed to be paying). But it would for a lot of people. Even for me it sucks, I had planned to use that money for something more interesting than papering over the holes in my insurance policy.
I can completely understand how people end up in bankruptcy. In my opinion, if the plan says it will cover 80% and I have to cover 20%, with a deductible and a maximum out of pocket expense for me annually, then thats what should happen for non fraudulent charges. I went to the hospital for a good reason. All of the dental work has been recommended to me by a general dentist, seconded by an periodontist, and concurred by the oral surgeon who actually did it. None of this stuff is me running up unncessary bills. So I have the max out of pocket put aside in an account every year, and all of sudden my bills blow that by 4x because they are declining charges, and I'm sucking money out of my general emergency account, it sucks.
I can handle it, I'm glad I planned for emergencies and saved money and live within my means, but I can understand how this doesn't work for people living paycheck to paycheck and its not their fault. Not long ago I was getting by with the max out of pocket in an account, no emergency fund, and saying if shit happened I'd put it on my credit card while I pumped all my spare cash into paying down student loans. So I'm just lucky this is all happening Jan of 2011 and not Jan of 2010. And anytime you can make a statement like that, it is hard to blame the people where the dates didn't work out for them.