There are a number of things that might have happened…the very cold, snowless December and early winter might have had a hand in it or possibly crown rot.

http://www.plant-care.com/iris-dying-check-crown-rot.html

There are also insects like borers that can do them in.

http://www.kearneyhub.com/news/local/bearded-iris-disappearing-with-invasion-of-pesky-macronoctua-onusta/article_66279e08-16fd-11e3-9e32-001a4bcf887a.html