What could cause a spine to curve forward, leading to a compressed lung and causing difficulty breathing?

Kyphosis. Osteoporosis is the likely culprit. Brittle bones crack and fracture throwing the spine out of balance, resulting in increased pressure in an asymmetrical fashion, leading to more cracks and more bending in the same direction. Scoliosis is often also present. Chronic intake of certain medications such as steroids can cause or worsen the problem.