I've thought about this for quite sometime. Here are the spots where I think it could possibly 'unravel'.
States with the largest cities (thus the most entitlement) are sucking the money out of the other states, via the federal government. CA, NY, NJ, PA, DC, IL, MA to name a few are bottomless pits for money.
The 'other' states could withdraw and form their own little territories. This would cause the maggot states to collapse.
The other scenario would be if one state (and I think it would take just one) started to refuse to fund programs mandated by the feds. If one would do it - I think it would spread like wildfire.
The other is government spending. I can see that bad spending decisions could make the U.S. more financially unstable than it already is. It may become apparent the "emperor has no clothes." Once the money is worthless, and the prestiege is lost - many states could decide they could do better on their own.