One-liner
When the largest economy's central bank moves rates, almost every other market has to follow. US Treasuries are still treated as the safest asset in the world, so when the US pays more to hold them, money would rather sit there than take big risk for a little extra.
Why it matters
You can be perfectly happy with your bonds and stocks until US rates start rising. Then other countries have to weigh whether they can hold their own rate, and the repricing flows into the of nearly everything you own (stocks are always marked to market).
Analogy
Small stones dropping into a lake barely touch the surface. But when the biggest stone on the shore falls in, the ripples reach the far side.
The catch
Assuming that investing in Brazil, or anywhere, insulates you from the US economy, that their problem is only theirs. Capital moves globally; the dominant central bank's choices reach almost everything.