If I worked remotely from MD for my three months of employment with a California-based company, why does my W-2 reflect a CA state income tax in addition to MD tax?

I received two W-2 forms from my employer, the only difference being boxes 15-17: one form reflected my MD income tax and the other reflected my CA income tax. Everything I've read so far indicates I'm only supposed to be taxed for the state where I worked (MD), not CA, but is that true? If so, was it an employer error to tax me for CA?