I've seen the same thing happen to a K21 2-up but at low-level but they had time to release and land ahead. BP - I imagine the force exerted on an open rear canopy during a winch launch would be enough to rip it from its hinges?
Might have been me you saw. Long Mynd in the early eighties? I was in the front seat; instructor in the back. The nose up pitch was substantial but didn't last long as the canopy self destructed, with apparently an impressive shower of perspex, none of which hit the tailplane.
In the early eighties I was merely a sparkle in my Dad's eyes!