Unless you take your car autocrossing every weekend, with the rack and bikes on the back, you're not putting the 'G's into the bikes you think you are.
Same for the 'soft but heavy' mystery force. If it was so soft that you weren't aware of it, it would have to have been to a very, very long time to put a bend in the tube. Possibly something very heavy was leaned against the bike, or the bike fell against something? It's possible that it could have been there the whole time.
I'm trying to prevent this from becoming one of those "correlation is causation" threads that seem to proliferate around here:. "Bike has bent top tube. Bike has been carried on rack; therefore, a Rack will Bend your Top Tube"