The unanswered question is why turn? If the winch cable breaks at 80m
almost by definition there's room to land ahead on a normal airfield,

You may not have realized that the two operative words in that sentence
are "almost" and "normal". Knowing nothiing about the particular field
that accident happened, I give the pilot the benefit of doubt and just
believe that he had his reasons.

brakes in that clip, so my guess would be that its a DG-500 rather than
a Jantar.

:-)
When the discussion started in that forum, they talked about a DG-500.
Later they corrected it to Jantar-1. Always according to that forum (I
know nothing about the Jantar), in that glider you can stall a wing by
applying ailerons, which perfectly explains what I see in the video. But
frankly, I don't think the tipe of glider matters.