Diagnosis
The pad brake is not engaging. On this model, the pad should stop within 2 seconds of releasing the trigger.
How to Fix It
This is almost always caused by fine dust inside the brake mechanism. Remove the battery. Remove the sanding pad. Use compressed air to blow vigorously around the spindle and into the gap between the spindle and the housing. Work the trigger while blowing it out. Reassemble and test.
If That Didn't Work
The brake mechanism itself, likely a small brake pad or spring, is worn or failed. This requires disassembly and replacement of the brake components.
Heads Up
Letting the pad free-spin increases wear on the bearing and is a minor safety hazard. Don't set the tool down until the pad stops.