Gelatin desserts don't hold up well when they're heated, for example. That's the thixotropic feature of the stuff. Jostled severely, its structure weakens. For all practical purposes, it becomes more liquid. Earthquakes enter the picture not because they make pouring ketchup easier for lazy people, but because all gels are not edible.
Thixotropic substances lie underfoot as well as on the pantry shelf.
Notorious among them is thixotropic clay, and probably the most notorious of the thixotropic clays is Bootlegger Cove clay. That's the slippery stuff that became instant underground mud during the Good Friday earthquake in 1964, turning many otherwise well built Anchorage structures into jumbled wrecks. One way to envision what happens is to imagine the flat little clay particles as constructing what amounts to so many tiny houses of cards. In the card-house spaces, water is contained, and the water actually helps support the microscopic structure. When the clay is shaken violently, as it was during the earthquake, the house of cards comes tumbling down, its movement eased by the lubricating effect of the now-free water.