How do flexor hallucis longus injuries occur?

See beloe: It is usually a hyperextention injury.

Flexor injuries: Flexor tendonitis is common with sports medicine injuries for example turf toe injuries. The tendons on the posterior compartment of the lower leg that insert to plantar aspect of the toes are easily injured in plantarflexion/inversion or eversion injuries. Treatment mostly consists of rest, ice, compression and elevation. Rarely surgery is indicated.