Well, you see… some sauces, like commercial ketchup & mayo, are examples of fluid gels.
Fluid gel may sound like a fancy molecular gastronomy term… which it is. But also it’s something that we, without knowing, use in our daily lives. So let me clear this up for you.
A fluid gel is a type of gel that acts like solid if left undisturbed, e.g. when ketchup sits in the bottle, it virtually becomes solid. But when this same gel gets shaken, mixed or otherwise disturbed it magically transforms into a liquid and flows smoothly from the bottle.
Et voila! Now you know why some sauces act like solids when they sit in the bottle for a while.