Dean Baker has a guess. And so long as we're talking offshoring, remember that the practice exists as much as a threat as it does an action. The specter of offshoring helps keep wages down, justifies cutbacks, forces longer hours, and all the rest. Indeed, I was talking to a Chicago community activist the other day who was recounting the story of Brach candy company. Brach was a commodity candy producer located in Chicago's Cook County. It was eventually purchased by an oddball Swiss billionaire who instantly set about crushing the unions, forcing down wages, and all the rest. For the first few years, he did this by threatening to move the company. This worked until some Chicago community activists commissioned a study on where the company actually could be moved. The study found that, at the time (this in the early-90s), no nations with cheap labor had the requisite talent base to run an advanced confectionary plant. The study was passed on to the city of Chicago, and folks stopped listening to the threats. But offshoring doesn't exist solely when jobs are actually flung across the world; it exists as a way to control labor domestically.
Postscript: Talent bases in other countries improved, of course, and in 2004, Brach moved to Mexico.