From the video, it appears as though the truck attempted to stop of the car ahead, but likely locked up the brakes, which resulted in the loss of traction.
The truck then veers right and loses control. It turns 180 degrees and head right at the Tesla in the nearby lane. This is when Autopilot kicks in and moves the Tesla out of harm's way. It all happens pretty quickly, so you may want to watch both the video above and the additional one below a few times to see what really occurs.
The Tesla owner, Chris Miller, reached out to InsideEVs and tells us the following:
I was on the 405 in traffic and a truck spun around 180 degrees and my car avoided it.
You can watch the video clip from another angle/camera below.