No one here on a message board can tell you what the new charge is for or why it did not arise earlier. You may want to speak to your son's attorney and ask him or her.
There are many possible answers to your question. There may have been an arrest for a misdemeanor that was recently filed. I am assuming that the defendant's attorney would have checked to have seen if there were any other open cases before entering into a plea agreement. I would contact the defendant's attorney and ask him what is going on with the case.
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