I believe they're going on the idea that the change was unconstitutional because it was a "revision" instead of an "amendment," and thus the California Constitution hasn't really been amended.
They'are also challenging whether it is retroactive, which is stupid since the California Constitution now says "Only a marriage between a man and a woman is valid or recognized in California." From the plain text even if you were married, the marriage is now neither valid nor recognized in California.
What really gets me here is the support of the CA AG for overturning this, including filing a brief with the court. This now being law, the government is supposed to be supporting it in court.