It's called raising because your tongue is actually in a higher place in your mouth when making that vowel. However, the "rule" for raising is different in Canada than those parts of the US where it's found. We raise the vowels before an unvoiced consonant, and leave the sound "low" before a voiced consonant. I think in Virginia it's applied to all instances of one of the diphthongs (can't remember if it's ai or au) whether voiced or unvoiced. The info is in my linguistics book at home, and I am, of course, at school...

I don't think they [the linguists] have generally concluded just why it arose. Certainly Canadians of Scottish descent seem to raise more strongly than others. And many of the first English-speaking Canadians came from Scotland. But we all* seem to do it.