Joe LaCava was Tiger's caddy for many years, and Rory and Tiger are very close. I can imagine those two (McIlroy and LaCava) know each other well. So how it happened obviously incensed McIlroy enough to not leave it to a quiet word afterwards. Of course the heat of the moment can make you lose your rag, but it carried on into the car park.
I think USA will rally in some games and will likely win the session but it won't be enough.