Jim:
No, bunkers usually get shallower -- the sand hardens or gets contaminated, and committees add sand to freshen them up. When they restored Seminole in the 1990's, they dug up something like six layers of old bunker sand, plus a layer of topsoil where the club had grown tomatoes during WW II.
On heavier soils, bunkers have a bottom and no one ever digs down past it. On sandy soils, though, it's possible for bunkers to get deeper, and on windy sites such as Bandon or in the sand hills it is an ongoing maintenance issue, although Sand Hills and Ballyneal have both adopted clever measures to reduce the sand loss over the winter months.