well how this happens is rather simple yet interesting
when there is insolvency going on with a country, or brutal change of government or planned war from others etc
what happens is that word of mouth spreads by bankers that this country is no good
suddenly like domino effect over night, NO ONE WANTS TO do business or even hear any excuses from the said country
and boom, overnight currency worthless
this is a very simple way to put it, but it does actually happen involving such a simple thing like phone conversations between bankers