They're not angry -- it's a display of respect. They're performing a Maori haka for their son (the guy with his back to the camera) who just got his first game time playing rugby for his national team, New Zealand.
It's actually a pretty cool tradition. If you look up "haka" on youtube you'll find a lot with larger groups. The guy in the foreground is on the rugby team called the All Blacks and they do a haka before each match!
They're not angry -- it's a display of respect. They're performing a Maori haka for their son (the guy with his back to the camera) who just got his first game time playing rugby for his national team, New Zealand.
It's a ritual that the Maori (native New Zealanders) practice. Seems rather insensitive and ignorant to condemn a harmless tradition just because it looks weird to you.