The title doesn’t match the article. It’s not about why toms descend in tone (typically - unless you are Jimmy Chamberlain and a handful of others). It’s about why toms have variable lengths.
I took it to mean the toms were descending towards the ground like stalactites descend towards the ground - they used to be shorter and now they're longer.
I guess the way they mean it is not about playing. It's about how, as your eye moves from one tom to the next, you see the bottom of the tom's shell getting lower.