No, he is not termed out as County Supervisor. He has greater political aspirations and I guess he believes that the assessor position would be a stepping stone to that higher position. Either that or he has become so unpopular in the first district that he may have trouble getting re-elected. Many people have questioned why he would seek a position that is considered of less importance than his current position as supervisor.
If he becomes assessor, I believe that he will give huge tax breaks to large businesses and developers.