There are a couple of things it could be. It could be the background was accidentally bent when it got put in and thus it is pushing up. Or, perhaps the device was not screwed together tightly enough and the background has some wiggle room. If water or some kind of moisture got into your Tamagotchi - by the factory or some other process - that would also cause the background to get wrinkled. In any event, warped backgrounds are very uncommon but it's nothing too worrying - unless its from moisture - as all virtual pets can work without a background. Some people even open up their virtual pets and swap backgrounds, which can look pretty interesting.