Buyers of bonds are starting to look irrational in terms of return. Some European countries are at negative rates and look likely to go lower. Germany just sold 5 years at a negative rate while Nordic countries have been there for some time. In the US bond yields are dipping lower than the Fed forecasts which creates the need for an adjustment by one party or both.
When thinking about what rational investors would be buying a negative yield or in the case of the US one that the Fed is projecting to be overvalued in a few years time. What I don't see alot of talk about is the time frame and the type of investors. While it is true that holding the new German 5 year to term is going to give you a loss on your money, holding the Bund could be a great purchase if you are a Greek investor with money in the Euro that you want to protect in the event of a Grexit, or if you feel that market turmoil mixed with asset purchases by the ECB will take yields even more negative in the near term. You simply think that Deflation cannot be tamed. On the US side you can have expectations that the Fed will blink first, citing low inflation as the cause, or a renewed interest in US bonds will remain popular for investors wanting to get out of Euro denominated debt, selling it to the ECB and buying the equivalent in a currency more likely to strengthen to the Euro than fall.
No matter what the reason is (reality is a mixed bag with a prevailing bias that is moving yields lower) anyone looking to invest in the space should be aware that most of these investors are not looking to hold to term and the "irrational drop" in yields could easily become a rational spike as the conditions bond holders expect are met.