There can be many different reasons why people result in acute or chronic pain and in my 30 plus years of practising as an Osteopath, I think I have seen just about all reasons.

My observations of the common reasons people seek Osteopathy therapy are:
Trauma (accidents such as falls, vehicle accidents, sporting injuries etc.)
Incorrect posture
Repetitive movements – creating strain and over-use
Prolonged positional holds – not moving frequently enough (common occurrence for desktop workers)
Illness (arthritis and more besides)