It wasn’t that the claims got higher, it was the theshold for damage by the manufacturers got lower. Used to be that 4% damage was OK (I think the number was 4%). Pushed by the import brands, Detroit pushed the RRs to reduce claims and speed shipments. This led to the current network of dedicated rack trains with flat switching (or racks moving in intermodal trains rather than merchandise trains) and the rise of mixing centers.
A secondary factor was the decline of body on frame and the rise of unibody cars. They were much more likely to sustain damage from the ties downs on a hard coupling.