How to stop treeview random calling after postback

You have to call populate treeview outside of if else condition and inside for loop.

Refer below code

private void PopulateTreeView(int parentId, TreeNode parentNode)
{
    TreeNode childNode;

    foreach (DataRow dr in _acountsTb.Select("[parent]=" + parentId))
    {
        TreeNode t = new TreeNode();
        t.Text = dr["code"].ToString() + " - " + dr["ac_name"].ToString();
        t.Name = dr["code"].ToString();
        t.Tag = _acountsTb.Rows.IndexOf(dr);
        if (parentNode == null)
        {
            treeView1.Nodes.Add(t);
            childNode = t;
        }
        else
        {
            parentNode.Nodes.Add(t);
            childNode = t;
        }

        PopulateTreeView(Convert.ToInt32(dr["code"].ToString()), childNode);
    }
}