Hmm, this isn't caused by HikaShop. You may want to consult the makers of your template and/or whoever has been setting up the site.
An explanation here would get quite lengthy, but in brief, there's a bit of "chaos" with the CSS files you site is loading, one of which is the proper default responsive one for Bootstrap v2.3.2, while others seem to have been added by the template (i.e. a Bootstrap v3.1.0 file, and a template.css file which appears to be modified).
It leads to at least (!) a "float: left;" missing for the various span* classes for screen or browser window widths in between 980 and 1199px, resulting in the categories stacking up vertically instead of displaying in a single row.
Yes, there could be a quick fix by adding a line of CSS code, but you may want to get to the cause of the "chaos" instead... who knows what else is all affected...