clumps? That's what is in the title.
Rocky soil will continually produce more surface rocks. It's called "granular convection" and there's really nothing you can do about it except continue to remove the stones as they come up. The winter cold and the water in the soil help this along. In the spring you will have a whole new crop of rocks. Through the summer the process slows down, especially if your summers are fairly dry. Just keep digging/raking them out and save them to make a rock border somewhere.