There is a simple bug when working with bootstrap style.
When using bootstrap, we can't add the default DataTable css. We only add dataTables.bootstrap.css.
But if using server-side processing, the "Processing" overlay text message will cause the table to slightly move downwards while processing, because the .processing css style is in the core CSS. This CSS basically makes the "processing" div to be absolute, making it to be on top of the table while it's being processed.

To fix this, I suggest adding the css below to dataTables.bootstrap.css as well.

.dataTables_processing {
    position: absolute;
    top: 50%;
    left: 50%;
    width: 100%;
    height: 40px;
    margin-left: -50%;
    margin-top: -25px;
    padding-top: 20px;
    text-align: center;
    font-size: 1.2em;
    background-color: white;
    background: -webkit-gradient(linear, left top, right top, color-stop(0%, rgba(255,255,255,0)), color-stop(25%, rgba(255,255,255,0.9)), color-stop(75%, rgba(255,255,255,0.9)), color-stop(100%, rgba(255,255,255,0)));
    background: -webkit-linear-gradient(left, rgba(255,255,255,0) 0%, rgba(255,255,255,0.9) 25%, rgba(255,255,255,0.9) 75%, rgba(255,255,255,0) 100%);
    background: -moz-linear-gradient(left, rgba(255,255,255,0) 0%, rgba(255,255,255,0.9) 25%, rgba(255,255,255,0.9) 75%, rgba(255,255,255,0) 100%);
    background: -ms-linear-gradient(left, rgba(255,255,255,0) 0%, rgba(255,255,255,0.9) 25%, rgba(255,255,255,0.9) 75%, rgba(255,255,255,0) 100%);
    background: -o-linear-gradient(left, rgba(255,255,255,0) 0%, rgba(255,255,255,0.9) 25%, rgba(255,255,255,0.9) 75%, rgba(255,255,255,0) 100%);
    background: linear-gradient(to right, rgba(255,255,255,0) 0%, rgba(255,255,255,0.9) 25%, rgba(255,255,255,0.9) 75%, rgba(255,255,255,0) 100%);
}