'da2a is not defined' error when using Headjs or Labjs

My site got a lot of script files so I want use the Headjs or Labjs to download them in parallel, hopefully for better performance.

However, after either of them is enabled, the AddToAny doesn't work, and the firebug captures an error of 'da2a is not defined'. When being hovered over, the 'Share' link behaves like it's dead and doesn't prompt the share menu.

The following is the difference I found before and after I enable the Labjs module:

• The share link

<a id="da2a_1" class="da2a_button" href="http://www.addtoany.com/share_save#url=http%3A%2F%2projectwhatever.dev%2Fnode%2F4003&title=this%3A%20problem%20is%20%&description="> Share</a>

<script type="text/javascript">
<!--//--><![CDATA[//><!--
var a2a_config=a2a_config||{},da2a={done:false,html_done:false,script_ready:false,script_load:function(){var ........................2a_config.tracking_callback=['ready',da2a.script_onready];
//--><!]]>
</script>

<script type="text/javascript">
<!--//--><![CDATA[//><!--
da2a.targets=[
{title:'node title for node 4003',url:'http://myproject74.dev/node/4003'}];
da2a.html_done=true;if(da2a.script_ready&&!da2a.done)da2a.init();da2a.script_load();
//--><!]]>
</script>

But when it doesn't work:

• The share link

<a class="da2a_button" href="http://www.addtoany.com/share_save" id="da2a_1"> Share</a>

The query string after 'share_save' disappear

<script type="text/javascript">
<!--//--><![CDATA[//><!--
$LAB.queue(function() {
var a2a_config=a2a_config||{},da2a={done:false,html_done:false,script_ready:false,script_load:function(){var ..................ge',{target:el});da2a.done=true;}}};a2a_config.tracking_callback=['ready',da2a.script_onready];
});
//--><!]]>
</script>

<script type="text/javascript">
<!--//--><![CDATA[//><!--
$LAB.queue(function() {
da2a.targets=[
{title:'node title for node 4003',url:'http://myproject74.dev/node/4003'}];
da2a.html_done=true;if(da2a.script_ready&&!da2a.done)da2a.init();da2a.script_load();
});
//--><!]]>
</script>

After I disable either of Labjs or Headjs, everything turns OK again.
What may cause this problem and is there anyway to get around this? Thanks.