My understanding is that Vanguard opened up a new institutional share class of their target date funds, a bunch of 401(k) money left the retail version for the institutional version, and the retail version had to sell a bunch of appreciated assets to facilitate that mass exodus. The remaining retail shareholders were then given a capital gains distribution, as required by tax law.
I don't think this is a risk unique to target date funds at all. If you're invested in any mutual fund that has a bunch of unrealized gains, and more people sell out of the fund than buy in during a given year, that fund is going to need to liquidate assets to pay out the people leaving, and that will lead to capital gains distributions paid to the remaining shareholders.
This can be a good reason to prefer ETFs over mutual funds for your taxable accounts, as ETF shares are created/redeemed in a different way that doesn't create a taxable event for the other shareholders.