Pedestrian bridge collapse in Florida

http://www.nzherald.co.nz/world/news/article.cfm?c_id=2&objectid=12013879

Does anyone know more about the causes of that collapse?

Now that we supposedly know so much about mechanics, why does a small-span bridge like that collapse?  

It it because we are optimizing prematurely based on false confidence in numerical models?

-- Biswajit