CA can only tax a non-resident on "California source" income. Here's a link to the California statute that defines "California source" income as it applies to a non-resident:
https://www.law.cornell.edu/regulations/california/18-CCR-Sec-17951-2
Note the wording "performed within this state".
I suspect that your employer corrected your withholding in mid-year so as to be in compliance with this statute.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
