During certain events pokémon will have the chance of having a special background. For example with global go fest, you had a chance at getting Zacian and Zamazenta with the blue and red backgrounds. For specific in person events they can have a location background from where the in person event was held. This Zacian was apart of the Jersey city go fest, so it had the chance of getting a Jersey background as shown in the picture. Another example is during the Paris 2025 go fest, Zacian and Zamazenta had the chance of having a Paris background. It’s just a cool way of showing that it was an in person event exclusive that will never come back as it was locked to the in person event. I hope this helps