DIAGNOSIS This is called "chain creep" or "coasting," and it's a serious safety issue. On this saw, it's almost always caused by a failing or stuck trigger switch or a problem with the electronic brake circuit.
FIX Immediately engage the chain brake by pushing the front handguard forward to stop the chain. Remove the battery. Do not use the saw. The trigger assembly is likely faulty. Sometimes debris like sawdust or a wood chip can get lodged in the trigger mechanism, preventing it from fully returning to the 'off' position. With the battery out, try working the trigger back and forth vigorously and blow out the area with compressed air.
IF THAT DIDN'T WORK The electronic brake function within the motor controller or the trigger switch itself has failed. This is not a user-serviceable part on this model. The saw needs to be taken out of service and repaired by an authorized technician. Continuing to use it is dangerous.
HEADS UP This is one of the few problems where you should stop troubleshooting and plan for a repair. A chain that doesn't stop when you tell it to is a major hazard. Do not attempt to bypass or "live with" this issue. The cost of a repair is far less than the potential cost of an injury.