What caused the population boom in California

While this is definitely vague, and multiple different things contributed to a large California population, I'd say that the gold rush was certainly a large factor, which is hopefully what you're looking for?

The California gold rush meant thousands of miners running to California trying to make it big, and boom towns were formed, increasing the population immensely.

Hopefully this is what you're looking for? x))

Answer: If you are talking about in the 1800's, the Gold Rush caused a population boom with everyone immigrating to the west to find gold.