What can be the reason of the crystallization of caramel?

Caramel is tricky stuff....so many things can trigger crystallization, even a piece of dust falling into the pot. Most recipes call for adding a little bit of corn syrup, which changes up the concentration of sucrose vs fructose in the sugar solution, thus preventing crystallization. You only add a small amount, so it doesn't change the flavor at all. If you try to speed up the crystallization process by supersaturating the solution (which is what you did at home...you were just lucky you got away with it that one time!), you're just setting yourself up for crystallization.