I once saw a chap 'PIO' his glider into the deck at Gutersloh right beside where I was standing
Glider PIO's can be due to CG out of range, eg someone accidentally leaving a ballast weight in or in my case on one flight, a loose trim tab that moved every time I moved the elevator. Too fwd a CG can lead to you running out of elevator authority and too rear can lead to too twitchy elevator!