I'm not a physicist by any stretch. Physics (and enjoying the pure logic of computers) is what drove me from electrical engineering to computer engineering. Having said that... I think it's all about the speed and direction of the air molecules you are riding in.

When you are riding alone, assuming there is no wind, the air molecules are just sitting there. You have to push them out of the way so that you can proceed. But when you're riding behind a bus, the air molecules are rushing to fill in the space that the bus just occupied and vacated. Those molecules, moving towards the vacated space, and away from you, are easier to push out of the way than are air molecules just sitting there, so there is less resistance. Less resistance means you can go faster with the same effort. But I don't think you can say anything is actually pushing you.