This sentence answers your question. Seems they were all asleep at the wheel...er...trigger. Too bad for the Iraqis killed.
I bet the guy driving the Toyota was charging the women and children inside big bucks to get them somewhere safe and so was driving way too fast in hopes of turning around and making the trip again that day. Sometimes greed can make people do stupid things. Sometimes boredom can make people do stupid things.
This sounds like a situation where lots of people were all doing stupid things at the same time. $hit happens.