What causes fallen arches? do arch supports in shoes prevent it getting worse?

To a degree: Arch is held up by static structures( shape of bones, ligaments) and dynamic structures( muscles of the foot and tendons). In a normal flexible foot arch flattens when your foot hits the ground, and the arch increases as you toe-off . An arch support is just that something that supports the arch. If it is falling because of a tendon rupture it is unlikely to prevent it from continuing to worsen.

Hereditary: Many people are born with flat feet, some diseases can affect the arches. Orthotics will sometimes delay and help the foot from getting worse.