Note that even as QE is being slowly reversed in the United States and the short term interest rates have been raised - the 10yr, 20yr, 30yr treasury yields are still tanking like crazy. So it is not the fault of the central banks, which is the point of the article.
In short - people are buying bonds which eventually drives the yield below zero. There's no rule that says "a bond can only be sold at below the levels that the 0% yield implies", therefore brace yourself for the possibility of breaching this level. For any currency, including USD.
But if the short-term rates are manually set by central banks to be artificially low, wouldn't that be the primary driver behind negative long-term interest rates even if the exact number is determined by supply and demand? The article is talking about natural drivers like "negative time preference" which just sounds wrong.
Central banks attempt to adjust interest rates to keep inflation at a target rate, which is a balancing act between maintaining a stable currency and stimulating (or resuscitating) an economy.