This car did exactly the same thing to me outside Reading on the M4 and did it to other cars before taking off. We were overtaking a middle lane hog and he undertook on the inside lane then he pulled violently in front of us (there was 2 car lengths between us and the car in front), tried to intimidate the driver in front by sitting on their bumper. When we came up to a lorry overtaking another lorry he dangerously undertook the lorry in the middle lane who had to brake when this car pulled in front of him. the guy is loopy!
A great driver!