It is because you have div tags with floated elements and min-height attributes and no clear after. There are quite a few places where you would have to correct this.
A quick fix is to update your css and add a clear:both to your .topGalleryContainer class on line 203 of your css file.
Then you will have to decrease your top margin for your infoWrapper on line 177
margin: 230px 316px 20px 0;
That will fix those buttons anyway.