It is being caused by an overlap of the <div id="bookmarksDiv">
Right now that div is 350 X 350 pixels wide whether it is in use or not and is always attached to the bottom left of the browser viewport. If you mouse over a link that is positioned under it, that link will not highlight because your mouseover or hover event is within that fixed div, not on the content under it.
You can see it illustrated much better if you use Firefox's Inspect Element function, it will give that div a border. Everything outside it functions correctly, everything under it does not function.
Unfortunately, because of the way your page is designed, I don't know of any way to fix the problem without getting rid of the div (and the social media links within it), or redesigning your site to ensure the links start 350 pixels from the left of the screen (so they are no longer under the floating menu div).