Many species of mushrooms seemingly appear overnight, growing or expanding rapidly. This phenomenon is the source of several common expressions in the English language including "to mushroom" or "mushrooming" (expanding rapidly in size or scope) and "to pop up like a mushroom" (to appear unexpectedly and quickly). In reality, all species of mushrooms take several days to form primordial mushroom fruit bodies, though they do expand rapidly by the absorption of fluids.
Given this paragraph about mushroom growth, why is it related to the English language expression "to mushroom"?
This is because many types of mushrooms exhibit rapid growth, seemingly overnight due to fluid absorption.