modal does not slide up on dismiss

[ray]

Very strange issue here.  When my modal appears, it slides down.  However, 
when it is dismissed, it does not slide up.  Instead, it seems to simply 
fade away, along with the backdrop.  Any idea what might be causing this?

Here's my HTML:
        <div class="modal hide fade" id="myModal">
    <div class="modal-header">
    <button type="button" class="close" data-dismiss="modal">×</button>
    <h3>Modal header</h3>
    </div>
    <div class="modal-body">
    <p>One fine body…</p>
    </div>
    <div class="modal-footer">
    <a href="#" class="btn" data-dismiss="modal">Close</a>
    <a href="#" class="btn btn-primary">Save changes</a>
    </div>
        </div>