The article didn't say, but it did mention NAFTA as a partial cause, so some of the traffic is apparently NAFTA trucks trying to avoid New York, Philadelphia, and Boston.
The author uses the term NAFTA because it is a negative buzzword among liberal groups, just like the term "smog-belching". It is an environmental pub, and the article has a good bit of editorializing in it.
While some truck traffic on I-81 has increased from NAFTA, most of it(including most of the growth) is still destined for the big cities of the Northeast. I-81 is indeed used as a bypass to avoid DC, often Philly and urban Jersey, and NYC for some. From Hagerstown north, and along I-78 east into New Jersey, it is amazing to see all the truck stops and rest areas at times chock full of trucks staging for their delivery times in the NYC area.