When the object slides toward the center, gravity has a component along the block’s velocity that speeds it up. After it passes the center and is moving away, gravity has a component that pulls backward along the block's direction of motion. As the block slides farther out on the plane away from the center, it is traveling “uphill” against Earth’s gravity, building up PE and losing KE. Eventually it comes to rest, slides toward the center again, and the process repeats. The energy changes are very similar to those occurring in a pendulum, with the total energy sloshing back and forth between PE and KE.