I would think that it is irritation in most cases. A prolonged ride down a rough road may cause new injury. There is no certain way to differentiate. It could just be that the rough ride trigger inflammation. That alone could bring on new or a return of symptoms.

I don't think it makes any difference regarding the healing process. Irritation slows the healing process. New injury, even if it is minor, effects the healing process. Getting caught up in the mechanism is futile. Even the best researchers struggle to understand the injury mechanism. They do recognize that there are a variety of injury mechanisms.