Edited - Field in Sight beat me to it. But I'll leave this post as it goes into a wee bit more detail.
Might it have to do with launch airspeed?
A very light glider will be trimmed for a low glide speed.
If you launch into a strong wind, it will tend to pitch up briskly, and any slight yaw at time of launching (ie your hand not holding it perfectly straight) would give a strong roll tendency - roll being the secondary effect of yaw, and slight yaw at high airspeed usually leads to quite strong secondary effects.
Strong pitch up would lead to loss of airspeed and subsequent pitch down. If the glider rolled at the same time, it would tend to end up in a descending turn, which may be a stable state depending on its design.
Compare this with a launch downwind or in nil wind - lower airspeed so aerodynamic response is reduced, and it would neither pitch or roll very fast. To the observer, the glider might appear more stable, and perhaps only slowly pitch / roll into a descending turn.
Hmm... Comments welcome on this theory.
Oktas8