This is something that happens from time to time on smoothing pads.
The base material for the smoothing pad is a synthetic felt. Once the phenolic resin is applied to that felt and allowed to cure, it slightly shrinks the base material. Felt materials (even synthetic ones) have variation in their production and their behavior. Synthetic felt is less erratic with a closer tolerance level of shrinkage than natural felt, but it will still sometimes shrink more than it should after curing.
This shrinkage will sometimes result in a pad curling up on the edges after water has been applied to it and allowed to dry. It is a simple fix of just rolling the edges of the pad back in the opposite direction. There is nothing wrong with the pad and it will not affect it's use by manually rolling back the edges.
You can also warm the surface of the resin with a hair dryer or heat gun to make it more flexible and bend it back to flat and let it cool.