I had that happen with an old Cannondale rack on my old Cannondale. My 1st 'tour', an S24O.

The front of the rack came unsecured and the rack, loaded with tent and panniers, rotated back until my Planet Bike SUperflash began to bear against the tire.

The tire chewed through the plastic on the light before I could get stopped, but a little duct tape kept her flashing for awhile.

Alas, she nonetheless grew despondent in her disfigurement and flung herself from her specially-crafted bracket and into rush-hour traffic on the ride home one cold, windy night. So it goes...

R.I.P, Blinkie