Your site was not loading the necessary CSS stylesheets for the page to render correctly.
So when Google tests your site it renders an unstyled page.
Without those styles, the pages HTML will be rendered with the browsers styles, which means:
some text will be too small
some links such as those found in menus won’t have the padding require to make space between them
some content won’t have a max-width to stop it from becoming wider then the page