Make sure you used array.
And define the default value using "data" attribute like :
...
...
->add('select_type', 'choice', array('choices' => array('1' => 'Value 1',
'2' => 'Value 2',
.....
...... ,),
'multiple' => true,
'expanded' => true,
'data' => array(2),
))
....
.....