I know exactly what you mean. One can only try to guess what it means to him. It is the kind of thing one sees with some 'thinking' bowlers. You plan and plot a batsman's downfall and it works and it is a personal triumph. I have seen something very similar from Prasanna. He was really one of those thinking and scheming bowlers. His celebrations were never over the top but when his mate and very close buddy Bishen Bedi was also in the playing eleven you should have seen them. They kept talking to each other a lot when either one or both were bowling. Then if one of them got a wicket, they would rush towards each other with those great big grins on their faces. Sometimes when they had really made a 'monkey' of the batsman they would burst into a guffaw. It was amazing to see at the fall of a wicket, their reaction as if only the two of them were there on the field of play. They were like schoolboys together enjoying the moment in which it was a joint victory for both every time. It was a delight to watch, never over the top, never offensive to the batsman and, of course, they never, EVER, said an unkind word to the batsman.
I don't know if it is a personal moment for Steyn as well who too comes out as a very intense individual who is wrapped up with his 'job' but I would like to think this might explain it to an extent. One doesn't know for sure, of course