In my experience, I've seen expedites been approved for people who are in the military (or work for the US government) and are being deployed/moving to another country.
Also, expedites when someone might age out of their category...
Then I've seen some expedites been approved for medical reasons but generally the person will not want to divulge the exact reason for expedite as it is personal.
Then others have noted they applied for expedite- it was denied the first time - but approved the second time (with additional evidence).
All you can do is apply for expedite and send in evidence that you believe shows the strength of your argument.