What causes costochondritis other than chest trauma?

Costochondritis: In most cases, we cannot track the initiating event, but it could be from excessive coughing, lifting heavy weights, strenous exercise, bad posture during sleep, skeletal deformity such as scoliosis. Infection is also a possibility.

Costochondritis: Usually there is no identifiable cause, but it can be caused by trauma, viral infection, or other inflammatory process.

Multiple causes:: Trauma being one that you mentioned; Others include: Inflammation from any reason; Infection, like Viral, bacterial, etc; Idiopathic, where no cause can be found for it. I believe a number of mechanisms work together and in most cases no particular cause can be identified.