I think it collapse for no reason whatsoever. A spontaneous collapse if you will. A catostrophic event which is in no way predictable or preventable, and something that can happen at any time and place to any kind of structure, regardless of it's construction materials, methods, design or modifications. Therefore no person is suitable to accept either responsibility or blame.
Perhaps this bit of sarcasm sufficiently argues that there IS a cause for all events like this bridge collapse. We just don't know the cause of this one.