The friction pads are being moved in such a way that they are trying to move away from the center. This is evidenced by the state of the clutch prior to the pads contacting the drum. Once the pads contact the drum, then the drum pushes on the pads to prevent them from continuing their motion. The force is generated by the cohesion of the material of the drum. To convince yourself of this, replace the drum with a strong elastic band. You will see that the elastic band will stretch to a point and then if it's strong enough, will hold the pads.