Not surprising at all
I had a similar incident here in LA a few weeks back.
As I was cruising about 75miles/hour in the carpool lane, typically for CA traffic, I approached another car in front of us going about 60 while there's nobody in front of them at all.
Being courteous, I patiently followed them at their speed for about 10 miles and then I got a little bit closer to their car (I'd say it was about 3 cars length) and then suddenly the yahoo stumped on his brakes which in turn made me to do the same and it scared the living hell out of my wife.
Guess what?
That driver took pride in what he did and raised his fist in a sign of victory.
What the heck!!!
So I waited until I could pull out of the carpool lane safely and legally to go around him.
When I got to be next to their car, I glanced over to find out who they are and the driver turned out to be some older guy (probably around 60s something).
That guy didn't even turn over to look at me and I thought about returning the favor but my wife said to let it go.
Here in SoCal, we have 2 different kinds of crazy drivers.
You either have some maniacs who would zip in and out of traffic going 95 to 100 or some idiots who would all the way to the left then start cruising at their own leisure and the heck with anybody else.