Brazil has restrictions against fuel surcharges, so there have been ways where starting in Brazil and routing on flights with low prices exclusive of high fuel surcharges has resulted in rather cheap tickets even more recently.

That said, sometimes people may say they choose a routing because its costs were low even when mileage/status running was part of the picture. And then there are the more deceptive reasons.

I see a trip like the one mentioned by FedGuard and I can see how it may have smuggler run written all over it -- perhaps even ingested contraband such as drugs or precious stones. That said, there are also people who legitimately do very convoluted trips to see more places just because the cost is right. So called fuel-dumping from Brazil makes perfect sense to me.