How did you try to reproduce it?

Did you create the grid first, and at the same time saving the returned gridObject for a second request?

The second request should look like this:

$.ajax({
url: '###',
type: 'GET',

data: params,
success: function(data){
gridObj.jqGrid('clearGridData');
gridObj.jqGrid(setGridParam, {sortorder: 'asc'});
gridObj[0].addJSONData(data);