Well, that was what surprised me so much. It was such a childish thing to do. And, yes of
course siblings quarrel. Heavens, my four siblings an I squabbled loudly and frequently, but never in public. What's in the family stays in the family.
But, what William did was very, very public. It was not just the people around that could see but judging from the sound on the cameras, there were many cameras, both stll and film recording their every word.
You don't get much more public than that short of taking out a front page advert so I am guessing whatever has pissed William off is important.