First note that sunnah has a meaning in Arabic which is:
As-Sunnah السنة is the route, way, norm, practice, line of conduct ... (no matter whether it is a good path or wrong).
Applied to the prophetic traditions it is defined as anything which is attributed to the Prophet Muhammad () be it a saying (statement), an act or action, an ...
Sabab is used as synonym for 'illah, and the two are often used
interchangeably. Yet the ulema of usul tend to use sabab in reference to devotional matters (ibadat) but use 'illah in all other contexts. Thus it is said that the arrival of Ramadan is the cause (sabab) of fasting but that intoxication is the 'illah of the prohibition in wine-drinking.