> But what I'm curious about is the causal difference between the two

A majority of personal bankruptcies in the US being caused by medical expenses might be a good place to start looking. You can be "broke" living paycheck to paycheck and "making it", but you're on even more of a razors edge than most. One medical emergency, one car accident, one removal of work hours etc and you start to fall behind, and that's when late fees and compounding interest work to make sure you never get out of the hole.