Please, pay attention that the ascending sort on the orderID is actually specified in the sortorder parameter. In general the default sort order of the last column mentioned in the sortname list is specified in the sortorder parameter and not in the list itself.
Now that sorting will work out of the box for local data only. It will just get the sort icons displayed on the columns for datatype xml or json. If you are using any of these two, i.e. server data, you'll have to sort your data on the server before sending it back to jqGrid. Luckily the sidx URL param contains the sort clause so that your server code knows which columns to sort on.