Here is the issue.
@Html.ListBoxFor(model => model.SelectItems, "CAT_ID")
The second argument should be the select list rather than a string, so it should be like
@Html.ListBoxFor(model => model.SelectItems, (ViewBag.CAT_ID as SelectList))
OR
@{
var list = ViewBag.CAT_ID as SelectList;
}
@Html.ListBoxFor(model => model.SelectItems, list )