The liquefied surface loses strength, causing buildings or other objects on that surface to sink or fall over.
In fact, the treacherous grit is rarely deeper than a few feet. Quicksand is not quite the fearsome force of nature that you sometimes see on the big screen. But no - the actual properties of quicksand are not quite those portrayed in the movies.
If you believed what you saw in movies, you might think that quicksand is a living creature that can suck you down into a bottomless pit, never to be heard from again.