Not possible the rack moved at only one point. So it does have a pivot. Like a Pletscher. And likely pivoted at the dropout as well. And maybe more places. Why would you own a rack that wants to be a deformable parallelogram? Did you think of immobilizing one or more hinges? And you rode up the hill without noticing anything amiss? Still would have stayed put if the p clamps were secure. You are making a compelling case for having your mechanical work done at LBS>