http://www.betterscanning.com/scanning/dismantling.html

In your case, the scanner is making a low Res preview and then when it's making the fine scan, the carriage isn't tracking straight.

Also I hate to mention the possible bad news. Bad stepper motor, they aren't in sync and the one side is working, the other side is just being dragged along, skewed.

Make a scan and look down as it previews, then make the actual scan, I think you'll see the light and carriage traveling at an angle.

Have some fun, take it apart!