Target date funds and other funds with target allocations (TAFs) rebalance in a predictable manner: when equities plunge, TAFs redeem bond fund shares and purchase equity fund shares, transmitting equity market shocks to the bond market. This introduces a new form of cross-market fragility: when equities fall and TAFs are expected to rebalance out of bond mutual funds, \textit{other} investors also redeem strategically, since liquidation costs are borne by the remaining mutual fund investors. These strategic runs are particularly pronounced among illiquid bond mutual funds, where liquidation costs are large and therefore payoff complementarities are strong. Together, TAF rebalancing and the associated runs explain half of aggregate bond fund outflows and a significant share of Treasury and corporate yield dislocations during COVID-19. The growth of TAFs contributes to the rise in stock-bond correlation and weakens the traditional role of bonds as safe-haven assets.