Looks like a shader at work. To describe it simply, the shader takes a region of the image and calculates an average color value out of it; every step, the size of the area is increased. Or more simply, the shader takes a color and repeats it across slowly increasing width and height, the colors just won't blend as well that way. Additionally, it appears that after certain amount of time they start changing the origin point to make it look as if it were zooming in, and crank brightness down. You say you've used the effect before but apparently don't know how it worked, so I assume you used an asset somebody else had made. If you still have access to that project you might be able to dig out the shader, which it almost certainly is as a shader is about the most effective way to go about this.