The Germans treated Soviet prisoners a bit like they treated jews and a huge pourcentage of Soviet prisoners ended up the same way : in death camps being worked and starved to death when they were not just shot down.

The Germans always pretended that the fact USSR never signed the Geneva convention meant they were not protected by it and that they could therefore do anything they wanted with Soviet prisoners.

It was in fact just an excuse to treat badly people they considered subhumans (jews were evil and manipulative, slavs were docile beasts of burden). The British / French and US prisoners though were relatively well treated as long as they behave, with adequate food and facilities and okeyish camps.

So it easy to see why they preferred to surrender to armies which had signed the Geneva Convention and whose prisoners they had treated fairly than to an army whose prisoners they had treated like vermin and that refused to sign the Geneva Convention.

Another fact that deeply enraged the soviet rank and file and made the matter worse was when they advanced in German territory and saw how well-off they were. They saw rich German farms with well-fed families that had a very comfortable life compared to the average Soviet citizen. They would have undestood if Germany invaded because they were starving and poor and invaded to try and get some ressources to fed their people, but when they saw the very good standards of living of German people first hand it really appeared like the Germans invaded, killed and brought all this suffering simply because they wanted even more even though already had a lot.

This fact infuriated the Soviets soldiers and they made their personal objective to take their revenge on Germany's population for their greed and cupidity. They gang raped the girls (some as young as 13), killed the husbands, burned entire villages etc... German prisoners were even more guilty as they actively fought for all this so retaliation on them was very important as well.

As a result a lot of German units conducted a fierce fighting retreat against the Soviets, then surrendered to the first British uniform they met, sometimes with slightly amusing results like the recount of the end by Guy Säjer in "The forgotten soldier" : the remnants of elite Panzergrenadier division GrossDeutschland leaving their defensive position with hands raised to surrender to a couple of astonished British soldiers in bicycles that were just scouting the area and stumbled upon the village the Grossdeutschland was dug in by chance.