Here is a good site:

http://www.webmd.com/a-to-z-guides/h...topic-overview

As you can tell lots of times it's your feet being crammed into shoes or heels and over time your toes can stay that way.

Another site:

http://www.mayoclinic.com/health/ham...SECTION=causes

As you can tell there are many causes.

This site mentions CMT, arthritis, etc.

http://en.wikipedia.org/wiki/Hammer_toe