We almost had a similar situation, but luckily got it corrected before we hit dumbass level, although I felt like a dumbass for a bit
We had strapped my yurt to the rack in a criss-cross pattern, making sure to go around the rack towers, but our almost-fatal mistake was that the straps compressed the towers toward each other - the force actually moved the towers (after-market Yakima rack with Q-clips on a VW Passat) toward each other, loosening the straps. About an hour from home I looked up and saw the yurt bouncing enough to concern me, we stopped and discovered the problem - had we gone much further we may have lost the entire load. We then ran straps from the towers across the front and back of the yurt, as well as the criss-cross we originally had, which pulled the rack evenly in BOTH directions. Once we did this, the thing didn't move a bit, 65 mph and it barely even lifted in the front.
Sambo - did your rack have metal clips that fit inside the door frame that bent, or did it attach to the car another way? I can't imagine the force required to bend Q-clips, but staying away from semi's is very good advice!