In my case, as a remote worker it was hard to say no to a house twice the size of my old apartment for a mortgage payment half of what my rent was. The only tradeoff was more rainy days, since I moved to a metro in the PNW.
Had housing prices been more reasonable in the parts of California where people want to live, I wouldn't have had any issue staying. It was nice.
A lot of it, as in 2020, came from the reduction in immigration to the United States (2020 and 2021 were about 300k below any time in the prior decade, due to pandemic-related restrictions.
California has had negative net domestic (within the US) migration offset by international immigration, for which it is a major destination for both employment- and family-based immigration, for quite a long time prior to 2020.
Another big chunk is retirements (which in 2020 and 2021 were accelerated by the pandemic); taking savings from working in California and retiring out of state to a lower CoL area has been common for a long time.
Probably an exaggeration to call it the leading cause, though, curtailment of immigration and accelerated retirements compounding the existing trend of taking retirement savings to move somewhere lower CoL are big factors.