The problem may also be the name, see here.
In the Controller
ViewBag.PersonList= new SelectList(db.Person, "Id", "Name", p.PersonId);
In the View
@Html.DropDownList("PersonList",(SelectList)ViewBag.PersonList )
This will not work, you have to change the name, so it's not the same.
@Html.DropDownList("PersonList123",(SelectList)ViewBag.PersonList )
So change yearDropDown and it Will Work for you.
Best regards Christian Lyck.
http://stackoverflow.com/questions/15686512/html-dropdownlist-selected-value-not-working-using-viewbag