Seems to have been just a sound change. The sound /r/ was lost before /s/ in words like curse (cuss), parcel (passel), and burst (bust). Sound changes like that are typically systematic, even if they don't happen to every possible word - words like worse, course, and fierce were apparently not included in this sound change. Fully or semi-regular changes like this and others are what create different accents, although sometimes a new pronunciation will get adopted with slightly different meanings into dialects that didn't go through the sound change - my dialect has both of the words in the pairs curse/cuss and burst/bust with slightly different meanings, for example.