a 标签触发器 href="" 时，为什么点击后弹出当前页面？

<div class="panel-group" id="accordion">
    <div class="panel panel-default">
        <div class="panel-heading">
            <h4 class="panel-title"><a href="" data-toggle="collapse" data-target="#aa1">标题一</a></h4>
        </div>
        <div class="panel-collapse collapse in" id="aa1">
            <div class="panel-body">标题一对应的内容</div>
        </div>
    </div>
</div>