D7 form field #ajax callback not called on autocomplete field
I think the problem is that $form['field_video_reference'] is a container and you want to attach the ajax event to the actual text field element.

If you inspect the $form array you should see the correct place to put it. In needs to be at the same level as the #type =&gt; 'textfield' (or entity reference) property.

I think the path will be something like:

$form['field_video_reference'][LANGUAGE_NONE][0]['value']['#ajax'] = array(
    'callback' =&gt; 'mymodule_video_ajax_callback',
    'wrapper' =&gt; 'my-path-div',
);