Bonds, and even fixed interest in general, only provide assured returns if held to maturity (and sometimes, not even then). If they are actively traded, there is no guarantee of low volatility or capital return.
For example, say you bought a bond for $x that would mature with a capital return of $y. Your expected capital gain/loss is already different from that of the original purchaser, who would likely have paid $y for the bond, expecting $y back on maturity. If you sell the bond prior to maturity, you might get $z, and all those prices are likely to be different.
A bond fund that actively trades its portfolio of bonds inherits this volatility. If it sees bond prices plummeting, it might decide to sell to cut its losses, but it also thereby crystallises its losses.