I really don't think someone just said "hey everyone we're all gonna plan during our SCRUM meeting today." My guess is everyone in the group already participated in some exercises (maybe the group has an "exercise" time when everyone does some basic working out which is optional but everyone has been participating). Then during one of the exercises (with everyone participating), someone throws out "let's all plank tomorrow during our meeting." If anyone just moans or says "nah" they scrub the idea.

Once we did a "hike" during our work meeting (this was my last job at a swimming pool so everyone besides the cashiers was very athletic) and everyone loved it.