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Not sure I understand your question? Bond funds/ETFs goes up and down because the value of bonds goes up and down. PLRIX behaves just like any other bond funds.
But if you're asking why bond values change.. let's say you bought an awesome bond with 10% rate. Then the rate changes, it's now 9%. So new bonds will have only 9%, while you still have your bond that pays out 10%. Your bond is now more valuable if you sell it. That's how bond value change over time.
Of course it's more nuanced than this. This might be of interest to you, if you want to dive in https://portfoliocharts.com/2019/05/27/ ... convexity/