I'm going to say it. It can't possibly collapse 20% in one day... can it?
There are examples of it happening in other countries though. Zimbabwe and Argentina come to mind for some crazy inflation and stock numbers.
Asset pricing is not continuous. A single trade can take pricing from -18% to -X%, with X having any value between infinity and -100.
The breaker triggers at -20%. If the market crosses at -18% and then trades -25%, that trade will cross and then trip the breaker.
Even the 1929 crash wasn't that much, though it did drop about 24.5% over two days, Oct 28 and 29, 1929.
https://en.wikipedia.org/wiki/List_of_largest_daily_changes_...
My understanding is that these automatic limit down rules were added in response to the flash crash in 2010, during which the Dow Jones lost 1000 points over the course of minutes.