Derailleur hanger moves when tightening quick release I have an old Trek T-900 tandem that my wife and I ride quite a bit.  It's got an 8-speed rear setup.

It's a standard (tandem) 145mm rear-spacing with matching hub, but tightening down the quick-release moves the derailleur hanger, screwing up the shifting.  I can re-adjust, but next time I remove the wheel, it shifts poorly until adjusted again.

This is an aluminum frame, so re-spacing it isn't an option.  Ideas of what is going on here?
Edited from my comments above:

The first thing I would check is whether the hanger is removable.  

If it is removable, I would check that it is still bolted securely in place. It's pretty common that removable hangers sort of overlap with the dropout. So it's possible that the hanger is loose, and the QR is then clamping it firmly in to place, but then each time you remove and reattach the wheel it clamps in a slightly different position.  That could explain the tricky shifting.