Why do "Machann lèt" and "Machann sik" become cuss words at times?

Machann lèt (milk merchants) and machann sik (sugar merchants), usually women, are not cuss words.  But in the flea markets, these merchants do have a reputation of being loudmouthed, temperamental and belligerent.  So, some people use it as an insult to call others by these names.