The truck was probably stopped and watching the pulse of traffic go around him (unless he was looking at the map trying to figure out why he was in the wrong spot).

When the traffic opened up, he decided to "go for it" as quickly as possible.

One thing they always say... if you can't see the mirrors, then the driver can't see you.

Anyway, it sounds like you were good, giving the truck some extra space. I suppose, if you were planning to pass it, then pulling to the left (or right) early would have been appropriate, which should have put you in the mirrors.