Does the B7 A4 audi not have pin points for the rack? I can't remember if my B7 had it but my B8 has points for the thule rack to "grab" into and I think it definitely helps keep it secure.
My guess is the distance between the towers was too wide, I had that happen the first time I put mine on and it definitely felt loose at a stop. At 65 I can see how if one side moved a little it would throw the whole thing off.
Is there anything you can claim under car insurance or homeowners insurance? Sucks that it happened, but I wouldn't not trust the rack, just constantly keep checking it. Good luck.