In summary, around 2006 PR entered a recession when all the factories started living the island and there was mass unemployment. The government continued the same level of spending, living outside their means got them in trouble. PR can not just make trade agreements with other countries and bring foreign capital in unless the US approves it first. So their hands were tied in increasing the economy in other ways. So stupidly they borrowed, and borrowed some more, this went on for multiple administrations. Now they have to pay and there is no money because the economy has been contracting for several years. Unlike the US, PR does not have gold mines, Oil, and other resources to sell and make money from, and it is unable by its status from making deals with other countries. A lot of people do not have the vision to invest for the future.
We did this to ourselves, both the people, the government and the directors of autonomous corporations after PR was pushed into a 10 years recession. Lots of people skimmed what they could and left other generations to pay. Little was used to upgrade the infrastructure. My Salud, which is free healthcare for the poor is a big drain since PR pays mostly all of the bills for that and gets less than states get for subsidizing healthcare. It is political suicide to mess with it and other social programs, so the cost is huge and they borrow to pay.
Lack of vision, a long recession, social services, inefficient systems, lack of inversion into the future, inability to trade freely with other countries, some stealing of the moneys, and many other factors have all made the problem grow.
Sorry I can not explain it in 140 characters or less.