You could set default sorting column to -1 by QHeaderView::setSortIndicator like this
yourTableView->horizontalHeader()->setSortIndicator(-1, Qt::AscendingOrder);
before running
yourTableView->setSortingEnabled(true);
because QTableView::setSortingEnabled uses QHeaderView::sortIndicatorOrder and QHeaderView::sortIndicatorSection to identify by which column should model be sorted.
Note that documentation of QHeaderView::setSortIndicator tells that not all models support setting sort column to -1 and may even crash in this case.