What are the biggest causes of hip fractures?

Falls in older folk. The greatest predictor for hip fracture is age. Older people are at much higher risk of fall, are more likely to injure themselves on falling and are more likely to have reduced Bone Mineral Density further increasing the likelihood of fracture. Specifically, physical frailty, reduced mobility, cognitive impairment, poor eyesight, medical morbidity, poly-pharmacy, predispose to falls in elderly.
Falls from standing. Older population - balance problems (due to medical conditions or medicines), poor eyesight, loss of coordination and muscle strength, slower reflexes - all these lead to increased rate of low energy falls that in combination with osteoporosis cause hip fractures younger population - high energy accidents (usually fall from height or motor) cause similar fractures.
Falls. Hip fractures most commonly occur in the elderly and occur as a result of simple falls.
Falls. Some are due to osteoporosis alone and break spontaneously. Others can be due to trauma such as car accidents. A smaller number are caused by cancer.