Why are parts of parent elements sliding with animate()? IE

Why are parts of parent elements sliding with animate()? IE

I'm having a bit of an issue with IE.

For some reason on this page:
http://www.rooxilist.com/bbnlev/innovation.html

When the right panel #content slides in, the background of the parent div #bgfield also slides in with it... Pardon my french, but WTF? I can't find anything linking the two. Is this some new weird "HasLayout" issue? Only a problem in flavors of IE. Halpplz!

The line of code for the slide looks like this:
$("#content").css({'margin-right' : '-651px' }).animate( { marginRight: "0px" }, 1200 );


The associated CSS looks like
/* Styles for Div holding the bg image */

#bgfield {
   position: relative;
   width: 900px;
   height: 448px;
   border: solid 10px #FFFFFF;
   /* background-image: url(images/transparent.gif); -- set this in each page */
   background-position: center top;
   background-repeat: no-repeat;
   background-attachment: scroll;
   text-align: center;
   padding-top: 112px;
   overflow: hidden;
}

/* Styles for main Content */

#bgfield #content {
   float: right;
   position: relative;
   background-attachment: scroll;
   background-image: url(../images/rightbg87pc.png);
   background-position: right top;
   background-repeat: no-repeat;
   overflow: hidden;
   height: 275px;
   width: 651px;
   text-align: left;
   font-family: Arial, Helvetica, sans-serif;
   color: #000000;
   margin-right: 0px;
}