Sometimes the fish dart about madly when the lights are first turned on, but then they settle down pretty quickly. Most will learn that the light is nothing to worry about & settle down after a while. I have no idea why they would all of a sudden became afraid of the light.
Someways to simulate a more natural "sunrise": Turn the lights on only after the room light is on, or after the tank had gotten some natural light. If there are more than one light, turn one on first, then the others. Put a diffuser over the light so it is not so bright. I see you have single tailed golds, & in ponds, they like to have some overhang & shade when sleeping. Perhaps you can have a piece of plastic covering part of the tank to give some shade, or maybe a big piece of driftwood or something in the tank for the same effect. <The fish are sleeping in the dark & the light might just startles them into a disoriented wakefulness. If they have cover to sleep in, they can swim out into the light after they are fully awake.>
One other thought: Goldfish don't really like white background. <The wild type is dark brown & I guess they still try to blend in to dark background.> Maybe the new background, being shinny & bright, together with the white gravel is reflecting too much light for them to feel comfortable. You might want to experiment with a dark cloth as your background & see if they like it better.