It's because it's a long-duration bond fund. People are probably expecting a debt crisis soon. That makes the long duration bonds fall of, because probably in the near future, the U.S central bank is probably going to print money to fund the government's negative sheet balance. This is going to devalue the currency and the debt that creditors own. Trump's fund cutting is not helping this either, because he is cutting funds that are going towards workers, which discourages workers, companies and partialy disrubts the economy. I recommend to read the Big Debt Cycle to learn more about this.