There's actually a great theoretical framework for this in an academic article (Xu & Yin, International Review of Finance, March 2017) from last year, but I'll summarize the basics here:
1. At noon, the ETF and the bonds are trading at the same price, call it $20. This is the old, stupid price, before we have the new information that things are awful. This is the equilibrium state.
2. At 12:01, there's new information that the market processes. $20 is now the "dumb" price, and there's a new theoretical "smart" price. Let's say that's $15. A 25 percent down day is a rout by any measure.
3. Market participants, who are always profit-motivated, will sell (and sell short!) at any price over the new, smart price. So they look for the opportunities, and lo and behold, here's all this liquidity available in the ETF! Inevitably, a huge amount of trading happens immediately, driving down the price of the ETF. At 12:02, the ETF is now trading closer to the "smart" price, call it $15.
Note that because the NAV of the fund — in this case, the intraday net asset value — hasn't moved yet, because in this moment, none of the underlying bonds have changed prices, and no bond services have issued new indicative prices for the less liquid holdings. So we'd now say it's trading at a discount.
4. Now it gets interesting. At 12:02, there's now this big discount happening, and a new set of market participants enter the fray — arbitrageurs. In this case, we have authorized participants, who know that they can swap the actual bonds for shares of the ETF, and vice versa. They now come in and do two things.
They start buying up shares of the ETF, while simultaneously selling the bonds, putting real prices in the market. This pushes the ETF trading price and the value of the bonds closer together, and slows the decline of the ETF in the process. At this point, the less liquid bond market comes online in full force, and the selling pressure on the ETF's holdings will begin in earnest.
5. By the end of the day, these counteracting forces and disparate market participants have had time to absorb the new information, and work out any friction from the system, and the ETF and the net asset value settle back into equilibrium at the new, smart price of $15.
It's also worth noting that, while the above straw man posits a sudden shock, we see this play out every day in ETFs. When lots of money wants in, ETFs tracking less liquid securities like junk bonds trade at a perceived premium until the market shakes out the noise. When lots of money wants out, the opposite happens: