- Many financial institutions are required to hold a certain percent of portfolio in safe assets. German bunds are among the safest in the world.
- A holder of a bond earns a capital gain (bond goes up in price) when interest rates fall. In that sense, zero is no limit at all because there can always be a buyer willing to accept an even lower (more negative) yield.
- Bond investors are well-aware of the two points above. When they sense that interest rates and/or inflation are headed lower, they know they can profit by buying, regardless of yield.
- Anticipated rate of inflation matters a lot because investors seeking return through yield focus on real interest rates (nominal rate - inflation). Inflation can be negative as well (deflation). If inflation is lower (more negative) than the bond's nominal return, that's a real positive yield. And that positive yield is locked in for the term of the bond, which in the case of the story is 30 years.
- The European Central Bank has repeatedly signaled its belief that zero is no barrier and that negative yields will be tolerated indefinitely. The ECB stands ready for quantitative easing (QE), in which the central bank buys bonds with money it creates from thin air. Investors know this and this compounds the incentive to pile on and buy bonds to enjoy the capital gains (and real returns if the investor believes that deflation is inevitable).
It's likely that all these factors combine to create the current environment. How long all of this can continue is anybody's guess because the situation is without precedent.
It's as if the financial crisis of 2008 was never resolved - just papered over through massive central bank purchases of treasuries and stocks (Japan's central bank owns a major fraction of the value of the Japanese stock market at this point).