My truck is wore out, and I can drive it straight. Come home and get in my Denali, and I weave. But only until I get used to it. Then I drive that straight as well. I think they're pulling the steering wheel because they maintain their lane always until the exact same point, and then they get all over the line.
But then again, you know how when you're passing a truck their wind pushes you sideways. So you have to fight the wheel to the right some to compensate, then the wind abruptly leaves, and you swerve a little bit. Maybe it's that. Because I only see it with vans and reefers. Never flatbed.