RSUs are taxable on vesting, and California wants its piece of that, if any of the vesting happened while you were a resident or working in California. But after they vest, they're just stock, and when you sell stock, California considers the capital gains to be sourced to your state of domicile.
If the OP was being loose with terminology, and the 'RSUs' to sell already vested, there's no California tax due on sale if they've moved out of state before the sale.
