California is quite aggressive at taxing income from stock people were granted while living in California.
In fact California specifically does not tax ex-residents for income arising from the disposition of stock acquired with ISOs, which is usually the way pre-IPO employees acquire shares. This is true even if the ISOs were granted for work performed in California.
The Silicon Valley company I worked at for a long time started out with ISOs, then as the company grew scrapped ISOs in favor of NSQ (nonqualifed options) and then later scrapped those and just gave RSUs.
The move from ISO->NSQ had some tax advantage to the company even though it is obviously worse for the employees.
It's a 1% tax hike on income over $1M, 3% on over $2M, and 3.5% on income above $5M.
I wouldn't call that enormous.
3% is enormous on top of existing sky high CA numbers...