So asteroid strike?
Although some scientists believe an asteroid MAY have started the 'trail' activity in Oregon 16+ million years ago, the question here is, is it, what they've thought for some time now, the North American Plate riding over a stationary deep-seated hot spot (like in Hawaii with the Pacific Plate), or is it an ancient oceanic plate still in the process of 'subducting' (an old ocean plate diving beneath the NA Plate) that is the cause of the line of volcanic erruptions. Both, I suppose, could produce a line of volcanoes, each younger than the one before, like with Hawaii.