So here's a quick primer followed by the obligatory apocalyptic prediction:
Prelude: cheap dollar financing
Pretend for a second that you're Brazil. Your economy is in pretty good shape and your currency – the real – is getting stronger. Because of this, people are willing to lend you money.
Your internal interest rates – that is, what you'd have to pay to borrow real – are around 6%.
But when you look overseas you notice that US dollars – which have been trending down for a while – can be borrowed for around 2%. So you run some numbers and conclude that if you borrow dollars and assume that the real continues to rise against the dollar, you'll make out two ways, on the spread between what you pay for those dollars and what you earn by investing them, and when you pay back the loans with depreciated dollars. So you borrow dollars, not just a little but a lot because with a lot you make a fortune.