I have noticed that my brother suddenly has a pronounced Philadelphia accent. Both of my grandparents had a Philadelphia accent but my mother has a much, much less pronounced accent and my siblings and I have accents similar to my mother's.
So why, WHY, does my brother has a sudden accent? Because it's in all of us and could surface at any time.
As an aside, when my lady was a girl her family went to Niagara Falls and when one of them asked for "water" in a restaurant up there, the waitress brought them butter.