Any two adults on one of those things would have that happen. It’s inevitable. Adults have a much higher center of gravity, and are heavier in general, than kids, for whom the equipment is scaled and for whom the spring resistance is set. It starts to fall over, and keeps going until someone loses grip, at which point the other person is launched regardless of relative weight differences. Its all about that spring.

This is an “adults on kid stuff are funny” thing. Doesn’t have to be more than that. Any adults who fuck around on playground equipment should have a general understanding of this, what with it being a near universal experience.