I believe the aircraft must have lost one of its engine and the pilot's weren't able to achieve zero side slip with one engine inoperative. Hence, the plane made a sharp turn increasing the bank angle towards the non-operative engine and lost all of its lift and stalled, which brought down the aircraft only 500 m (AGL) above ground level.

When aircrafts like Dornier twin loses an engine, it loses 80% of its climb performance, due to the increased drag and decrease in excess power required for a climb.

The pilot's main concern here would be is what is called asymmetries. Because each engine on either side of the plane would be providing equal thrust (therefore pulling the aircraft in a straight line), the loss of propulsion on one side may cause the aircraft to yaw in that direction. While not a catastrophic thing, the pilot does need to act upon it with opposite rudder to keep the plane flying relatively straight.