If the lead car hit the brakes for no reason, he has been negligent and would be liable. This could happen if you hit the wrong pedal, or presumably if your AEB

If the lead car hit the brakes for no reason, he has been negligent and would be liable. This could happen if you hit the wrong pedal, or presumably if your AEB system malfunctioned and thought it saw someone in the road. It might also apply if a squirrel ran into the road and you mashed the brakes to give it time to scamper away. Most people would say it is negligent to create a substantial risk of human bodily injury to avoid the possible/likely death of a squirrel.