From the article:
Eyewitness Jerry Kocharian, 62, said he was standing outside the Church in the Canyon drinking coffee when he heard a helicopter that was flying unusually low and seeming to struggle.Engine trouble?“It wasn’t sounding right,” he said, “and it was real low. I saw it falling and spluttering. But it was hard to make out as it was so foggy.”
The craft vanished into the sheet of fog, and then Kocharian heard the boom and saw a fireball.
The twin P&W turbo-shafts (jet engines) on that helicopter are known to be reliable engines. He was in SVFR weather conditions and off-course (departed towards the mountains from his intended course over Hwy. 101)..."controlled flight into terrain" (CFIT) or maybe a tail rotor failure are more likely possibilities.
Fuel starvation?