But if you go back further why did that happen? All the great society programs of the 60s are what ran up the debt. The notion that we're so rich we can just government program ourselves out of very thorny problems by giving people money. I'm not necessarily saying that doesn't work or isn't good. But it does cost a lot.
Why did we think we could do that? The new deal maybe? I don't know. It didn't bankrupt the country and it did give people jobs.
You could probably keep going back further if you wanted. Presuming that you know the exact date and time that the fall was precipitated is fun but not intellectually honest. Everything tends to build off of things that came before until it all falls apart.