[brc]

Is there a general reason why, to an outsider, most South American currencies seem very unstable and periodically afflicted by very high inflation?

[Sherlock]

The "Original Sin" of not Governments not being able to get debt in the same currency they are able to collect taxes.

So, when the main source of USD suffers in some sense (export prices, or some kind of supply shock) the Government (unable to get funds) must retort to fiscal policy to meet its commitments. And fiscal policy is the number #1 recipe to get (hyper-)inflation