This was a typical terrorist frame-up. They hide among civilians and then attack, which in itself is a war crime. Then they blame the people they attacked for civilian casualties. From the pardon statement:

These veterans were working in Iraq in 2007 as security contractors responsible for securing the safety of United States personnel. When the convoy attempted to establish a blockade outside the “Green Zone,” the situation turned violent, which resulted in the unfortunate deaths and injuries of Iraqi civilians. Initial charges against the men were dismissed, but they were eventually tried and convicted on charges ranging from first degree murder to voluntary manslaughter. On appeal, the D.C. Circuit Court of Appeals ruled that additional evidence should have been presented at Mr. Slatten’s trial. Further, prosecutors recently disclosed—more than 10 years after the incident—that the lead Iraqi investigator, who prosecutors relied heavily on to verify that there were no insurgent victims and to collect evidence, may have had ties to insurgent groups himself.