Joined: Jun 2013
Posts: 4,094
Likes: 2
From: Bozeman
Bikes: 199? Landshark Roadshark, 198? Mondonico Diamond, 1987 Panasonic DX-5000, 1987 Bianchi Limited, Univega... Chrome..., 1989 Schwinn Woodlands, Motobecane USA Record, Raleigh Tokul 2
There is absolutely nothing on the frame that could cause the ROTOR to move back and forth unless the caliper was physically loose and sliding back and forth.
Your rotor is warped. Fix it or buy a new one. My guess is you whacked it when installing the rack. (Or dropped the wheel on the disc brake side.)