I travel a lot, and I find my luggage is most often damaged in the U.S. by United Airlines and its affiliates.
I've had brand new wheels broken off, side handles ripped off, zipper tabs ripped off with such force that the remaining part was bent and unuseable, zipper compartment torn halfway off, the list goes on.
We're not talking cheap luggage either, often it's a Samsonite that is destroyed. The customer service related to damaged luggage is pretty rubbish too, I might add.
I'm always early to the airport, luggage is clearly labeled and not overstuffed, etc. There's no logical reason for some of the destruction except as swisspea described - unlucky positioning as the baggage went through the automated system and stuff got destroyed. I've not (yet, touch wood) had any luggage damaged outside the U.S.