The GOP loves playing "chicken" with the debt limit.  First, they run up the debt 5 trillion under Trump, and then they "find religion" and become the stewards of fiscal responsibility and hold our country's credit worthyness hostage.

On the surface, this looks like a battle between the 2 parties, and negotiations will resolve the showdown.  But it might be more dangerous than that.   

It is possible this is also a showdown between the Freedom Caucus (GOP crazies), and the adults in the Republican party.  Given McCarthy has probably made concessions to the right-wingers it seems more likely that the unspeakable result of the US Default actually happening which would cause huge economic damage to the world economy.  Since McCarthy made the concession that any one congressperson can call for a vote on his leadership, the crazies have outsized power.

If the chances were 3% in years past, I would put the chances at 15% presently.