Unlike this rare incident and this rare incident, this latest rare incident took place in Iraq.  The motive is unknown, and no pattern has yet been observed.  By Prashant Rao for AFP:

BAGHDAD (AFP) - Two American soldiers were killed on Tuesday when an Iraqi army comrade opened fire after an argument over a sports match, the first US deaths since Washington declared an end to combat operations here.

The shooting, which also left nine American soldiers wounded, happened at the Iraq's Al-Saadiq Air Base near the city of Tuz Khurmatu in Salaheddin province while a US army company was visiting local security forces.

"Iraqi soldiers and American military advisers were playing sports when a quarrel broke out between an Iraqi soldier and an American," defence ministry spokesman Major General Mohammed al-Askari told AFP.

"The Iraqi soldier opened fire on them," Askari said, naming the gunman as Soran Rahman Saleh Wali.

"The American soldiers killed the Iraqi soldier. We have opened a high-level investigation into this issue."

A US military statement said: "Eleven US soldiers were engaged with small arms fire, killing two and wounding nine, inside an Iraqi army commando compound."

The gunman was a member of one of the army's elite special forces units, said Colonel Hussein Bayati, police commander for Tuz Khurmatu, north of Baghdad.

There were no details on what set off the argument or on the Iraqi soldier's possible motives.

However, Bayati said that on Monday, US and Iraqi forces "began searching houses in the neighbourhood where this soldier was from because they suspected Ansar al-Sunna (insurgent) fighters were hiding there."

It was unclear whether Wali might have already been under surveillance or if the sweep had angered him.

[...]

Under the terms of a bilateral security pact, American soldiers are allowed to return fire in self-defence, and take part in operations if requested by their Iraqi counterparts.

Thank goodness U.S. negotiators were able to push through the self-defence clause.

[...]

"This is a tragic and cowardly act, which I firmly believe was an isolated incident and is certainly not reflective of the Iraqi security forces in Salaheddin," said Major General Tony Cuculo, US commander in northern Iraq.

There is also the case of Kaissar Saady al-Juboory, a soldier in the Iraqi Army who shot 5 US soldiers in December 2007, killing Capt. Rowdy Inman and Sgt. Benjamin Portell.  The Association of Muslim Scholars, a Sunni group, claimed that the US soldiers were beating a pregnant Iraqi woman, and Kaissar Saady al-Juboory shot the US soldiers to protect her.  Quoting the Association of Muslim Scholars:

"His blood rose and he asked the occupying soldiers to stop beating the woman.  Their answer through the translator was: 'We will do what we want.' So he opened fire on them."

Quoting Sheikh Juma' al-Dawar:

"Kaissar is from the al-Juboor tribes in Gayara -- tribes with morals that Americans do not understand.  Juboor tribes and all other tribes are proud of Kaissar and what he did by killing the American soldiers. Now he is a hero, with a name that will never be forgotten"

The US military denied reports of the beating of pregnant women, and said that Kaissar was a member of a Sunni insurgent group.

There was also an incident April 6, 2006 in which an Iraqi soldier shot and killed U.S. Marine Lance Cpl. Bryan N. Taylor inside a coalition base near Al Qaim in Iraq.

UPDATE: The shooter in the most recent incident has been identified as Soran Rahman Saleh Wali.  His brother Marwan, who works as a policeman in Tuz Khurmatu, has been arrested.