Nobody knows. The government has already said the subject is closed, after admitting they don't know. That's a shame, because the same kind of brains that built the electrical sytem are required to maintain it, as well. That means natural curiosity and an active interest in what causes faults.
This is why Vault-Co thinks we're on the brink of a new Dark Ages. When people are asking, "Why ask why?" it's probably because they are incapable of answering those kinds of questions anymore. Feeble minds + complex infrastructure = failure.
Vault-Co regulars are probably willing to try a hypothesis, I could think of a couple. Holes in the magnetic shield will knock out juice faster than you can ask somebody to light a candle. Brazil better hope this isn't the start of a trend because most of the people there are not capable of keeping this stuff running once it starts to burn out on a regular basis.
UPDATE : You won't believe this - submitted by a reader
When I suggested this last year about the Quantas flight, it was tongue-in-cheek speculation and I only got the idea from reading Sorcha Faal. As it turns out, now air safety wardens are seriously considering the possibility.
I figured we'd see all kinds of strange anecdotal news stories as a result of random fracturing of our magnetic shield - "Sheepherder in Brazil claims Mystery Rays from Outer Space Burned Off His Testicles," that kind of thing, National Enquirer tabloid style stories with kernels of truth in them. Maybe a real farmhand on a high plateau in Argentina developing weird radiation sickness while out on his land for a few days. Somebody who looked at the broader picture would then start to see these stories were all really about the same thing. I still believe this. I advise you to check the "hotsheets," to quote Tommy Lee Jones in MEN IN BLACK.