The reason is simple, when the Fed increases interest rates, investors will flock to new bonds with higher yields, therefore the older bonds' prices will fall. Bonds with longer maturities will fall even harder because you will be stuck with its lower rates for longer time.
What are the target date funds doing to minimize such risks?
Many target date funds are seeing this risk coming and shifting longer maturity bonds to shorter ones. But this means lower yields which is a different risk to people, especially the ones who already retired, holding target date funds.
To compensate the lower yields, some measures target-date funds turned to higher yield junk bonds, which this will increase default risk for the investment. The silver lining of this risk is, when the Fed raises interest rates, it typically means the economy is healthy, which means high yield junk bonds' default risks will be subdued.
In short, there is no easy solution, which could be a reason for investors to reevaulate their target-date fund investment decision.