Figures 22 & 23 attempt to show how moving from a downdraft to an updraft can increase angle of attack. Aerodynamically any sudden change in vertical air movement does change angle of attack. Add a sudden tailwind component and you may well be stalled as I have found out in my glider.
In my case I was downwind of a ridge in a 15 kt wind aloft having just turned final. Second hand I heard that another pilot had a similar encounter quite a long time ago.
The rest of the time, people fly the approach without incident.
The crew may have crossed this ridge with a similar flight path many times in the JU52 and other aircraft, but this time the thermals combined with a wind coming from a certain direction with a certain speed that upset the applecart.