I know very little but my guess is that this happened because of maths. Maths is always to blame. Shall we abolish maths?

Well afaik math is very useful. Without it, all countries will get crisis, right?

And you know what is quite funny? The World's Largest Debtor Governments leader is US with debt of 18,540,448,667,000. Second place is UK with 9,590,995,000,000.
We truly entered age of debt. Puerto Rico's debt is like one grain of sand on a desert compared to US debt. Does this means US will be defaulted in the future to?
Source:
https://en.wikipedia.org/wiki/List_of_countries_by_external_debtSeems US external debt in per capita still lower than UK, Germany, Netherlands, even Luxembourg.
http://www.forbes.com/sites/pascalemmanuelgobry/2012/10/19/no-the-united-states-will-not-go-into-a-debt-crisis-not-now-not-ever/