There are countless reasons as to why the court date was moved. There could have been a clerical mistake or perhaps your case has not been filed by the DA or City Attorney yet. To speculate as to why the court date was changed would just be a guess.
Do you know if charges have been filed by the prosecuting agency yet? I would call the DA's Office or the City Attorney to find out if charges were filed.
In my experience, the most common reason they push back court dates is because the CHP is not ready to turn over reports to the DA's office. This could be because they are waiting for blood tests or simply because they have a backlog of cases they are working on.
Some agencies like the CHP are responsible enough to actually send you this courtesy notice. Other agencies don't bother and have you show up to court only to be told "no filing yet" and to check back. I've seen many clients not follow up and be suprised when they have a warrant. It's a good idea to check once a week.
But to answer your question, it usually has to do wth a backlog of reports on the part of the arresting agency.
best of luck,