Thank you for reporting this problem.
I have been able to replicate the problem in a test case and have logged a bug report with the developers.
Until the bug is fixed, the only workaround I have discovered so far is to stagger the start of each gallery's AutoPlay by using the Juicebox-Pro API toggleAutoPlay() method instead of setting autoPlayOnLoad="TRUE" and then delaying the firing of this method for each gallery with setTimeout functions of increasing values.
Here is what the embedding code might look like for three galleries on the same page:
<script>
var jb1 = new juicebox({
baseUrl: 'g1/',
showAutoPlayButton: 'TRUE',
containerId: 'juicebox-container-1',
galleryWidth: '400',
galleryHeight: '600',
backgroundColor: '#222222'
});
jb1.onInitComplete = function() {
window.setTimeout(function() {
jb1.toggleAutoPlay();
}, 200);
};
</script>
<div id="juicebox-container-1"></div>
<script>
var jb2 = new juicebox({
baseUrl: 'g2/',
showAutoPlayButton: 'TRUE',
containerId: 'juicebox-container-2',
galleryWidth: '400',
galleryHeight: '600',
backgroundColor: '#222222'
});
jb2.onInitComplete = function() {
window.setTimeout(function() {
jb2.toggleAutoPlay();
}, 400);
};
</script>
<div id="juicebox-container-2"></div>
<script>
var jb3 = new juicebox({
baseUrl: 'g3/',
showAutoPlayButton: 'TRUE',
containerId: 'juicebox-container-3',
galleryWidth: '400',
galleryHeight: '600',
backgroundColor: '#222222'
});
jb3.onInitComplete = function() {
window.setTimeout(function() {
jb3.toggleAutoPlay();
}, 800);
};
</script>
<div id="juicebox-container-3"></div>