Welcome to the forum, Dave! My first thought would be flex in the machine. Are your cuts too aggressive for your machine? What feeds/speeds are you running? Is this a climb cut or conventional? What bit are you using? Is that side of the rectangle straight in your CAD program?
Oh, and we do like photos! They always seem to help in diagnosing an issue.
Oh, and we do like photos! They always seem to help in diagnosing an issue.