Dividends paid out. It happens every December for all mutual funds.
If your 401(k) reinvests dividends, look at the number of your shares. You will have an increase in the number of shares comparable to the % drop in the NAV.
You didn't lose money. You just exchanged the value of the dividends paid out for additional shares.
ETA: And by 'dividends' I mean capital gains distributions paid out as dividends. It's an IRS requirement that mutual funds pay out any capital gains the fund had during the year as dividends so that the fund owner will pay taxes on the gains.
You would see those distributions which were paid out on the 1099 statement you would receive if
your mutual funds were in a taxable account. Since it's in a retirement account, you won't get a 1099 and you won't pay taxes (now) on the distributions.
This post was edited on 12/6 at 10:12 am