Your jQuery set your element in absolute
position.
you can test and find out using !important
to override some of the CSS set by jQuery: http://jsfiddle.net/t6B2e/9/
#B{
height: 200px;
width: 100%;
background: blue;
/* overrides any inline style or javascript style */
position:relative!important;
bottom:0!important;
top:auto!important;
}