My wife and I recently returned from a vacation that took us to
Pennsylvania, Maryland, and Washington DC. While there, I noticed that
many trees and shrubs are planted on top of small mounds (about 2-3 ft
in diameter and 1-2 ft high for a shrub, larger for a tree).

No, these were not planted "at grade" and then soil mounded around them.
I could tell by looking at the base of a tree or shrub that a mound was
formed first and then the plant was placed at the top of the mound.

Why is this done? We don't plant things that way where I live. Here,
some trees and shrubs are actually planted in a slight depression (an
"anti-mound") to catch water and prevent run-off during irrigation.

--
David E. Ross
Climate: California Mediterranean
Sunset Zone: 21 -- interior Santa Monica Mountains with some ocean
influence (USDA 10a, very close to Sunset Zone 19)
Gardening pages at http://www.rossde.com/garden/