In CA who is at fault if a driver gets rear ended due to a sudden stop as he is switching lanes?

More information would really be needed to determine who would be At Fault. Generally, the driver of the car that collided with another car is responsible for the accident. They were following too closely if they were in the lane the driver was switching from.

However, if the driver of the car that was hit was switching lanes aggressively (in a manner that could have been considered to be reckless) and was rear ended by someone who was in the lane they were switching to they may at least share in the responsibility.