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.
A person can ascend stairs, or descend.
A drummer could play an ascending pattern on their toms, or a descending pattern.
But neither the toms or stairs have an inherent upward or downward direction.
It’s possible the title is a bit of a pun, and refers in part to both tone and shape.