Towpilot fatality in Oregon

I once *almost* caused a tug upset flying an Olympia 463, also only fitted
with a belly hook. Aerotowing towards the ridge at Talgarth, the glider
was suddenly pushed up by either thermal or ridge lift. The glider
continued to go up relative to the tug even though by now I had the stick
hard forward against the stop. This was after the accidents mentioned
below, so I realised what was happening and released immediately. As it
happened, the piece of knotted twine that Talgarth used as a weak link at
the time also broke at the same moment, so we lost a pair of Tost rings.
If found, please return to the club!

How does that work, was the towplane not in the same thermal or ridge
lift?

-Paul