I've read a discussion on the grammar of this a while ago.
Basically nous-autres and vous-autres correspond to the emphatic forms "moi", "toi" and "lui", particularly in topic/comment construction (because "nous" and "vous" lack separate object forms): "nous-autres, on sait cuisiner", "vous autres, je peux pas vous sentir" etc. The extension to eux-autres is a natural analogy. (English does not have this construction, which is close to cleaving)
I do not know what the specific etymology is (and @Oct's seems like some awfully pop-whorfian approach to me), and the etymology of many grammatical constructions is not very well known anyway. There is a sociolinguistic element to it, since it's not a formal construction, so it tends to be marked as low-class.
I went and looked at my Grevisse (14th ed.), with the caveat that it is an analysis of mostly written, European French (though it does often discuss regional usage). The relevant sections are §358 a), discussing adjectives applied to pronouns, and 659 h), on reinforced pronouns (autres discussed in the same context as -même). In both instances there is a brief mention of opposition/separation, but he also notes that there is a marked correlation with topic/comment-life construction, which in the Quebec dialect/sociolects we are discussing, will actually require the -autres forms.
I think a sensible argument is that what was originally an emphatic form (remember that this topicalization is already an emphasis!) was generalised to topicalizations as a whole (and a few others: "chez" can also sometimes be used with -autres pronouns without cleaving): a proper cleft sentence "C'est à nous-autres qu'ils l'ont donné" is also valid.