There were two major incidences before the killing of the Blackwater employees. The first is when the US forces shot into a crowd of protestors killing many of them.
The more significant one which was overlooked by the media is that the US forces raided the house of local Sheikh when he wasn't home and took his wife to the local US Army camp so that he would have to show up in person (Sheik in this case is a tribal leader not a religious leader).
Of course Fallujah (and Western Iraq in general) has quite a tribalistic community where tribal laws and arbitration procedures trump State as well as Islamic law. So when the wife was later released she spoke about how the soldiers entered the home without knocking on the door and that she wasn't wearing Hijab at the time which made the Fallujians furious. It was not only about her been seen with out a Hijab it was more about invading a home when the male household wasn't around. Something the Americans knew nothing about.
The rest of the story goes without telling. Kafirist forgot to mention that their bodies were heavily mutilated and dragged through the streets.
The next thing the US Army did is that they sent some 100s of Marines to raid homes and detain any male over the age of 18 regardless of the facts. Grievances grew among the community leading to many attacks on the US forces. Hence, the battle of Fallujah.
Now I depise Blackwater and everything it has done in Iraq. I lost a colleague in the
Nisoor Square Shooting. But that doesn't mean you should hang corpses on bridges. That's barbaric.