FYI: There's not a lot of mystery to a stock fund's returns. Take May 2019, which was a turbulent month for equities. What contributed to Vanguard 500 Index's (VFIAX) 6.4% decline that month? To dig into it further, we'd look at the returns of each stock the fund holds and consider each stock's weight in the portfolio (the market value of the position divided by the portfolio's net asset value). The fund's three worst-performing stocks that month-- Intel (INTC) (0.9% of net assets), Bank of America (BAC) (1.1%), and Apple (AAPL) (3.7%)--each lost around 12% to 13%. But because Apple was more than 3 times as large as the other two positions, its 12.4% loss detracted more than 3 times as much as the others'.
Understanding a bond portfolio's performance drivers, on the other hand, is a much trickier undertaking. For starters, there's no widely available resource where you can look up monthly returns for individual bonds, like you can for stocks. One way to get a rough sense for what's behind a bond fund's performance is to compare the returns of relevant bond-market-sector indexes and consider the market-value-weight of a portfolio's exposure to those sectors.