I have a theory about this. I don't think it's folks being jerks most of the time. Instead, I think there are one of two things going on:
1. Your faster speed, whether you're passing or catching up to them, is a conscious or unconscious reminder to them that they are going too slow.
2. There is a bond of sorts that is created when you get close to them. Passing fast and suddenly breaks the bond or reaching an absurd speed too fast to maintain the bond (ie they become aware how fast they're going) breaks the bond.
I find myself doing this sometimes and proactively have to cancel or defeat it so I only go the same speed I was already going. However, most of the time I don't allow myself to be the slow, passive vehicle on the road; instead, I'm proactive, but not necessarily FAST, just faster.