jesus christ people, dont read complexity into this.
2 guys talking in a park, they are talking very close to eachother, indicating that they don't want whatever is being said being heard. This would be suspicious to your average old lady. For whatever reason they were being filmed, that's the difficult part.
Why the chick got hit is incredibly easy, look at the film:
0 sec's : guy is pointing off in some direction, his conversation involves pointing.
The slap is not a slap at all, look again. she gets hit with his fore arm.
It's simple, the conversation involved another pointing gesture, the guy turns around and points his arm out, oblivious to the chick being there. Look at his demenor afterwards, hes just as suprised, he may even be stoned.
The bearded guy didn't look at the couple, he was checking out the chick just like you, I and every hetero man would.
The guy in the athletic suit has obviously beat some ass before. He reacts quickly and makes short work of the guys. His woman reins him in and they walk away.
Beared guy does not want to fight, his reation is "hey hold on a minute" BLAM "dont touch me Gesichtharrmann!"
When it's over, the two guys regroup and realize what happend, and they go after the couple again. what they intended to do, i do not know. If they wanted vengence, they would have ran. They may have wanted to clear things up, either with a "WTF was that, it was an accident" or "We're so sorry, dont beat our asses every time you see us!"
they catch up and the guy is not happy to see him, and begine beating, woman cries out to him to stop.
summary, the cause was:
Not paying attnetion -> roid rage ->not knowing when to leave it alone -> roid rage.