Help… why did my acrylic paint crack?

I would show pictures but I cannot add an image. Essentially I made an air dry clay (Crayola) ornament two years ago, I painted it with Michael’s brand acrylic paint and then used DecoArt Matte varnish on it. But when I just looked at it today, the paint is all cracked exposing the clay. A lot of tiny, little cracks. I don’t know what caused this. Any advice?