C # All of my comboBox are changing text

I'm having a problem with my program in that when I change the values of one of my comboBox , example:

private void Ex1Mat_SelectedIndexChanged(object sender, EventArgs e)
{
    if (start == false)
    {
        SqlDataAdapter Ex1UpdateAss = new SqlDataAdapter("select Assunto, id from dbo.Assuntos where Materia = " + Ex1Mat.Text + "", conex);
        Ex1UpdateAss.Fill(AssTable);
        Ex1Ass.DataSource = AssTable;
        Ex1Ass.DisplayMember = AssTable.Columns[0].ColumnName;
        Ex1Ass.ValueMember = AssTable.Columns[1].ColumnName;
    }
}

They all change their DisplayMembers what in case it was to change only one.

Do you have any idea what this is?