Ehhhhh...as someone who frequently drives on California freeways, I'm going to just throw something out there:
They were probably driving somewhere around 80+ to begin with, and traffic was probably moving decently (this is quite common--I was going 95 on the 110 earlier today for a few miles). Traffic probably started to slow, and he realized they were having a hard time slowing the car down. He probably hit the brakes hard, and then continued to press harder and harder as the brakes wore down and eventually started shooting flames because they were shot. At this point, the car is speeding out of control, and is flying by at a rate of about 120mph...or 1 mile every 30 seconds. Panic has probably set in, and the call to 9-1-1 is made as the officer frantically tries to just avoid hitting the much-slower traffic because he knows he would probably kill them if he did. Not knowing how to turn the car off probably threw him for a loop, and in this state of confusion and panic didn't think to throw it into neutral.
That's how I'm guessing it went down. Also, as to why he didn't steer into the center barrier, well, the 125 doesn't have a concrete center barrier. Being a more rural freeway, its lanes are separated by a wide drainage ditch flanked with a standard wooden post guardrail and wide left shoulders. The overpass structures are also separate structures for each travel direction with a gap between, so just another reason steering toward the center would not have been adviseable.