If you are thermaling, the glider is not operating anywhere near a structural limit like any max G or speed. Flying an inattentive slow circle makes for the stall and subsequent spin. (Ask me how I know.)

So why not just fly out of the unusual attitude? Was there another glider, or not enough distance to the ground? Or perhaps there was a problem with the recovery?

It's possible the glider inverted for a moment going well past VNE.

This video is an example:
https://www.youtube.com/watch?v=DPWaNxTP-SA