It was the Model S with "FSD" turned on according to driver that came to a rapid stop on the left lane that initiated the chain reaction. The video showed that it was going at least speed limit on 2nd to left lane. Signaled a lane change into left most lane and came to complete stop rapidly. There was already a car in the left lane that didn't expect such behavior and there just wasn't enough following distance created to react fast enough to avoid hitting the Model S. Tesla's are well engineered except for the "FBS" software-sensor suite and the latter is so because of marketing hype by Musk.
The lead car was the Tesla using FSD mode which apparently stands for Full Stop Driving Mode. Stopped in the left lane of a CA freeway is a death wish.
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