I also was surprised at only 2 landouts in over 1,000 flights - certainly not a path I seem to be following!
However it can be very easy to get caught out if there is wave about - the Cu works wonderfully well under the 'up' and then one hits a patch under the 'down' and can go from well above to well below the glide slope remarkably quickly. It helps to have a suitable field already picked... 'A massive amount of sink' does fit with wave interference, and the general area is certainly subject to wave.
The other thing the report doesn't mention is if the glider pilot had enough height to do enough of a circuit and land in the same direction as the other traffic.