What kind of behaviour is that?

I was driving on the middle lane the other day with a normal speed, 70-80 like tat, then suddenly I saw this car speeding, coming towards my direction, then with a very fast speed, cut into the lane beside me, which is the fast lane. After that, the car slowed down and drives slowly. When i said slow means, it's really slow, maybe 30-40 like tat!!

The car behind him kept flashing at him, asking him to get lost, but the driver chose to ignore and happily driving slow at that lane. I just do not understand why he wants to do that? purposely drive so fast to cut into that lane, then slowed down and get cursed by the cars behind him?? hmm.. perhaps he enjoyed getting cursed and flashed by other people? i guess i will never understand these ppl. ~sigh