There can be many reasons why a cat's back arches, such as:
1. The cat may have just woken up. At this time, the arched back indicates that it is very relaxed and has been sleeping comfortably. It is equivalent to stretching and there is nothing wrong with it.
2. It's also possible that the cat is bored and wants its owner's company, so it's doing this to get the owner's attention. In this case, if the owner isn't too busy, it's recommended to play with them for a while.
