Wilfred, you know that a stall develops when the airflow can no longer flow smoothly over the wing upper surface. Remember the airflow speeds up over this part. As you climb, the increased speed of the air gets towards the point where compressibility starts to have an effect, and once shock waves start forming the airflow begins to separate from the wing = stall. Used to be known as a 'shock stall'.
It becomes easy to stall a wing in a turn in these conditions, as the airflow speed further increases due to the increased lift generated by the wing.