In PivotMatrix you can click an icon to expand all items for any fields. Perhaps you can write a js function to evoke that icon's click once PivotMatrix is ready like this:

<script>
    function expandAllUsers() {
        //look for Users' Expanding All icon and call click
        ...
    }
</script>
<?php
PivotMatrix::create(array(
    ...
    "onReady" => "function() {
        expandAllUsers();
    }"
));
?>

Be noticed that PivotSQL is not the most efficient for expanding all nodes because each node expanding requires one sql query executing.