Microclimates due to terrain effects, mostly in the western areas(orographic lift/rain shadow) largely determine winners and losers. In areas at lower elevations it is random in a lot of cases, but there are localized boundaries that can help trigger storms due to bay/sea breezes enhancing lift in some cases. When a legit cold front runs into a warm, unstable airmass the chances for more widespread coverage of storms is increased, but there will still be locales that maximize more than others. Again over time, that seems to even out. A linear/elongated line of storms(QLCS) probably gives the best chance for all in the affected area to be impacted in a similar way.