Hi,
It's probably because something uses jquery on your website without calling the noConflict function:
forum.joomla.org/viewtopic.php?t=283215
Normally, adding a call to it where jquery is used just after jquery is added to the document should solve the problem:
$document = &JFactory::getDocument();
$document->addCustomTag( '<script type="text/javascript">jQuery.noConflict();</script>' );
It's a well known incompatibility problem between mootools (the js library of joomla) and jquery which some components/templates use. It creates problem in the initialisation of mootools on the page which breaks the modal box used in HikaShop. Unfortunately, we can't do nothing on our end.