Excessive load on the foot from obesity is a major cause of plantar fascitis. Which is why this condition is common in middle aged and over weight adults.
A sudden increase in weight, such as pregnancy can also lead to plantar fascitis.
A sudden increase in walking or a sporting activity can also be a contributing factor. A classical example is a post man who has returned to work after a period away from the job can develop this condition.
Tight plantar fascia (this is often caused by tight calf muscles).
Excessive flattening of the arch on weight bearing.
Biomechanical problems (walking abnormalities) is a major cause of plantar fascitis.
Different types of arthritis can also lead to this condition such as osteoarthritus and rheumatoid arthritis.
The plantar fascia also has an important role in dynamic function during gait . It was found the plantar fascia continuously elongated during the contact phase of gait. It went through rapid elongation before and immediately after mid-stance, reaching a maximum of 9% to 12% elongation between mid-stance and toe-off.  During this phase the plantar fascia behaves like a spring, which may assist in conserving energy. In addition, the plantar fascia has a critical role in normal mechanical function of the foot, contributing to the "windlass mechanism". When the toes are dorsiflexed in the propulsive phase of gait, the plantar fascia becomes tense, resulting in elevation of the longitudinal arch and shortening of the foot (see 3A). One can liken this mechanism to a cable being wound around the drum of a windlass (see 3B); the plantar fascia being the cable, the metatarsal head the drum, and the handle, the proximal phalanx .