There are so many reasons for it, for example, if a country has some political clashes between their politicians or something in between their country's politics, then this should be also a reason.
The other reason is a relationship with different countries, which means if a country has a strong relationship with a country and between them there comes a clash, then the exchange of their export and imports between the countries stops. So, this should be also a reason.
The third reason is the country's increasing population if the population of a country increases then they don't want to pay their dept.
Because there will be more spending in a country on their health care, the security of their country, the pensions they are offering to its country's people, etc. So, I think these will be the reasons for the countries which have, but they didn't want to pay. There will be many more, but I have discussed some here.
When there is a strong economical relationship between countries, even if war starts, business still stays the business. For example, my country had war with Russia, lost its territories and people died but still, there is a very strong economical bond because we are neighbors and get cheap food and natural resources from Russia.
Btw I don't understand this debt thing: Greece, Italy and Spain have one of the highest debt burdens but people from my country immigrate to work in those countries while my country isn't in debt burden. This makes me to think that debt isn't as bad as they say. The only country where life is bad is Brazil from list.
Isn't your analysis of wartime economic linkages insightful and revealing? Russian events demonstrate that business and economics often shape international relations differently from politics and military. Maintaining business relations during wartime is difficult
Next, we discuss national debt, which confuses even economists. How can countries with massive debts offer immigrants jobs and opportunity? The diverse character of economies answers your question. High debt levels may be bad, but do they prevent a country from providing for its inhabitants and immigrants?
Despite fiscal problems, Greece, Italy, and Spain have infrastructure, industries, and services that need labor. Isn't that why immigrants come? Despite debt, labor is sometimes more tempting than scarce prospects in a country with less debt. Navigating global economy and individual chances is tricky, right?