I think that was the key to all this. South African crowds are brutal, much more so than England or Australia where there tends to be more underlying humour.
Warner had a tour of the crowd saying things about his Mrs, holding up signs, wearing Sonny Bill masks, and so on. The South African players also pull no punches when it comes to sledging.
I think it was as simple as he copped such a psychological barrage that he just snapped. Had a melt down and lost the plot trying to hit back at them, got to a younger team mate who worshipped him and out they went with the sandpaper.