If/when it does, it will likely be because of erratic behavior on the part of the government (especially the executive branch), rather than the absolute magnitude of the debt itself.

Which is certainly high, especially given that the current administration made a hatchet job of cuts and yet increased total spending. But the country is still very productive, and has more than sufficient GDP to cover the interest. Which is all that's really required; the principal of the loan can roll over indefinitely.

That said... current total revenue is about $5.5 trillion. Taking out $2 trillion off the top begins to put a strain on the ability to pay for everything else.

So the house of cards is not on the verge of crashing down. But the government could well knock it over.