I just wanted to say hello.

I return the favor with a good old fashioned one finger salute.

Some possibilities:

1. They are being jerks. It's their way of sarcastically saying, "Thanks for moving over, you lowlife pedestrian!"

2. They know they are cutting you off, and they are half-apologetically waving as if to say, "I know I'm being an asshole, but thanks anyway for letting me cut you off, you're such a nice person and that was really kind of you."

3. They really wanted to say hi to you, and went out of their way to cut you off to do so. Highly unlikely though.