Argentina didn't decide. It couldn't pay the interests anymore. It was a snowball of crazy high interest rates in a foreign currency. You can't pay your debt if it's much higher than GDP growth even with a surplus. And Argentina had neither growth nor surplus, thanks to the idiotic IMF economic plans with a currency pegged to the US dollar.

A better point might be, why were they lent money in the first place?!?

Debt to GDP ratios can also be described in terms of how long it takes to produce GDP equal to the debt. Owing 100% of GDP is owing one year's product. Whether that's a significant dividing line depends on rates of interest and growth.

And to a de-facto government.