I read a brief memoir of Joseph Mironovich Yampolsky, a Red Army tank commander. He describes looking through German POW's for SS after their surrender at Stalingrad, and makes this statement:
One more thing struck us: almost one in ten of those wearing German uniforms were former Red Army soldiers. Summary execution was applied to them also.
Why were Red Army soldiers fighting for the Germans in German uniforms? Was this occurrence as common as he implies?
I wasn't aware of it being that widespread, but a significant amount of soldiers were recruited from occupied Eastern Europe by the Germans, including Russians, Ukrainians, and Belorussians. They were known as the Ostlegionen/Osttruppen. Often, they fought for political ideology, whether they believed in fascism or if they were simply angry at the Soviets. They included, for example, some members of Stepan Bandera's Ukrainian insurgents, as well as former White Russians. One particularly famous member of the Osttruppen was this guy: http://en.wikipedia.org/wiki/Yang_Kyoungjong
That being said I find it odd, because my recollection is most Ostlegionen troops were stationed as garrison troops and/or on the Western front (manning the Atlantic Wall, for example).
It was very common. There were hundreds of thousands of them.
At Stalingrad, the German Sixth Army had over 50,000 Hilfswillige ("Hiwis"), with the 71st and 76th Infantry divisions having more then 8,000 Hiwis each. In 1944, a Germany infantry company of 147 men had 19 Hiwi positions. A report in June of 1944 counted 287,400 Hiwis on the Eastern Front.
While not supposed to be front-line fighters, they often had roles that involved them fighting (assisting in the operation of crew-served weapons, for instance) and when needed, pitched in with personal weapons when necessary because capture meant death. They were also cooks, drivers, medics, and security in rear areas, although the vast majority of them were not normally armed.
Why did they fight? Motivation can be as individual as the man, but there are several obvious reasons:
Survival. The survival rate for Soviet POWs was abysmal, with more than 3 million of them dying through mistreatment. A Hiwi received rations and medical care like anyone else in the Wehrmacht, albeit you were usually treated with anything ranging from disgust to amused contempt.
Nationalists/Secessionists/Extremists. Some were politically motivated, either from a hatred of the Bolsheviks, a desire for their region/country to gain independence from the Soviet Union, or your average right-wing extremist. The Nazis, despite their distaste for "Untermenschen", were forced to realise they needed to turn at least some of the extensive population against the Soviet Union even if they had to paste a more acceptable label on the resulting units, e.g. calling them Cossacks.
It happened. Fighting and living in Stalin's USSR was not exactly the dreamlife and it is easy to see why some of the Red soldiers would envy the German soldier's life of being well fed, well equipped, well trained and generally well commanded.
The purges had eliminated a lot of very competent officers from the Red Army that had been replaced by fanatical members of the party that were often not competent and greatly enjoyed the huge all out charges with the communist flag flying high. Keep in mind also that being made prisoner by the enemy could often result in getting sent to gulag when getting back to USSR as the general consensus was that you were supposed to fight to the death for USSR, getting captured therefore meant you had failed your duty.
You would be exempted from gulag only if you could prove that you were wounded at the time of your capture and that you therefore did not have the physical ability to fight to death.
Another dimension was that red army prisoners were treated basically the same way as the jews, so executed, tortured and worked to death without much food.
It was therefore easy to see why some of them would be tempted to join special German troops such as the Dirlewanger SS Brigade which recruited basically the lowest of the low (common law criminals, rapists, pedophiles, sociopaths, deserters etc...) to send them to fight the partisans and put terror in the local population.
Desertions in this kind of unit was of course high and mutiny were generally avoided by making sure these guys were drunk most of the time and had a lot of occasions to indulge their sickest perversions on the local populations.
The Brandenburgers (special forces commandos, specialized in infiltrations and coup de main behind the enemy lines) also recruited some eastern european soldiers but the profile was then very different. They were not looking for thugs and sociopaths like Dirlewanger but for very motivated and professional soldiers that spoke well allied languages for behind the enemy lines commando operations. The Brandenburgers also did not accept fanatical nazis as the soldiers composing this unit would have to be able to fight well alongside soldiers from anywhere, including subhuman races according to the Germans, and be able to pass themselves as enemy soldiers which was not very compatible with political fanatism.
So basically it was not common but it definitly happened.
Christian Ingrao, The SS Dirlewanger Brigade : The history of the black hunters.
Eric Lefebvre, Brandenburg Division : Commandos of the Reich (Special Operation Series)