It could have been on the car for 4-5 months without anything on it to create upward pressure. Strap a board to it and you have an instant sail putting some pretty strong forces on the rack.
No aerodynamics expert but if the issue had been the boards creating a sail effect, I would imagine that the leading edge, ie the front
rack would have lifted off.
OP -- did you retrieve the rack after it detached? I'd be on the phone with Thule, if there was a manufacturing defect I believe they'll step up. If it was the wrong foot pack (the part that secures to the car) that, sadly, is on you.
In the end though, it feels like installation error.