Tabs at middle of page

Why does clicking on the tab move us to the top of the page?

HTML

This is the top of the page.
<br /> <br /> <br />
<br /> <br /> <br />
 <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br />
<br /> <br /> <br />
<div id="div-container">
  <div id="div-tabs-container">
    <ul class="ul-tabs">
      <li>
        <a href="javascript:void(0)">Tab 1</a>
      </li>
      <li>
        <a href="javascript:void(0)">Tab 2</a>
      </li>
    </ul>
  </div>
  <div id="div-highstock-container">
  <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum</p>
  </div>
</div>
<br />

CSS

#div-container {
  width: 1080px;
  overflow: auto;
  background-color: #FFFFFF;
  padding: 10px;
}

/* Style the tabs. */

#div-tabs-container {
  background-color: #FFFFFF;
}

/* Style the list */
.ul-tabs {
  list-style-type: none;
  margin: 0;
  padding: 0;
  overflow: hidden;
  border-top: 1px solid #ccc;
  border-left: 1px solid #ccc;
  border-right: 1px solid #ccc;
  background-color: #f1f1f1;
}

/* Float the list items side by side */
.ul-tabs li {
  float: left;
}

/* Style the links inside the list items */
.ul-tabs li a {
  background-color: #CCCCCC;
  border-right: 1px solid #ccc;
  display: inline-block;
  padding: 14px 16px;
  transition: 0.3s;
  color: #000000;
  font-family: Verdana, Arial, sans-serif;
  text-align: center;
  text-decoration: none;
  font-size: 17px;
}

/* Change background color of links on hover */
.ul-tabs li a:hover {
  background-color: #FFFFFF;
}

/* Create an active/current tablink class */
.ul-tabs li a:focus, .active {
  background-color: #FFFFFF;
  color: red;
  border-bottom: none;
}


/* Style the highstock and legend. */
#div-highstock-container {
  overflow: auto;
  background-color: #FFFFFF;

  margin: 0;
  padding: 0;

  width: 1080px;
}