What are the causes of hammer toe/claw toe?

Irritation : Tightness of the tedons to the toes and rubbing and irritation to the joints cause the joints to enlarge and the toe will contract and not straighten causing a hammer toe.

A couple: Hereditary or genetic is one. When the flexor tendons (the ones on the bottom of the toe) over power the ones on the top of the foot the net effect will be a hammering or clawing of the digits.

Multifactorial: A hammertoe is an abnormal contracture or a bending of the toe joint which results from a muscle or tendon imbalance due to the mechanical and structural changes in the foot that may occur over time. This can be hereditary or due to environmental factors such as tight shoes.

Bent toe.: People can get hammertoes several different ways, including just an unlucky roll of the genetic dice, but the fundamental result is usually the same: a toe that has drawn back and no longer sits straight or "flat". Sometimes it's only a tendon that holds the toe in that position, other times there's arthritis involved: these tend to be more rigid, and require surgery if you want them corrected.