DIAGNOSIS The sanding pad adapter is not fully seated or is worn. The DCS354B uses a bolt-on sanding pad, not a quick-change.
FIX Remove the pad. Clean the tool's blade mount and the back of the pad adapter. Ensure the pad's drive hole aligns with the tool's pin. Reinstall, making sure the pad adapter sits completely flush before tightening the blade bolt firmly.
IF THAT DIDN'T WORK The plastic sanding pad adapter can warp from heat or wear. Inspect the plastic for cracks or a worn drive hole. You need a new sanding pad assembly (DeWalt part number DWA4210 or similar).
HEADS UP A wobbly sanding pad creates uneven wear, ruins sandpaper quickly, and reduces control. Do not use it until it's solid.