Warum zeigt meine Twitter Bootstrap 3-Site mobile Elemente in IE8 auf einem Desktop an?

<header>
    <div class="topArea clearfix">
        <div class="container">
            <div class="topLinks">
                <div class="btn-group">
                    <span class="flag" data-toggle="dropdown"></span>
                    <ul class="dropdown-menu">
                        ...
                    </ul>
                </div>
                ...
                <div class="topNav">
                    <ul class="hidden-sm">
                        ...
                        <li>
                            <a href="#">
                                <button type="button" class="btn btn-default">Where to Buy</button>
                            </a>
                        </li>
                    </ul>
                </div>
                ...
            </div>
        </div>
    </div>
    <div class="mainNavArea">
        <div class="container rel">
            ...
            <div class="navi">
                <div class="navbar">
                    <div class="container">
                        ...
                        <div class="nav-collapse collapse navbar-responsive-collapse">
                            <ul class="nav nav-justified">
                                ...
                                <li>
                                    <a href="#">
                                        <button type="button" class="btn btn-default">Where to Buy</button>
                                    </a>
                                </li>
                            </ul>
                        </div>
                    </div>
                </div>
            </div>
        </div>
    </div>
</header>

.container {
    max-width: 960px;
}

<!--[if lt IE 9]>
  <script src="../../assets/js/html5shiv.js"></script>
  <script src="../../assets/js/respond.min.js"></script>
<![endif]-->