That's really unfortunate to hear. I think everyone can be forgetful when they stop, but usually I see this with small groups of inexperienced adults who thought they'd go cycle on a lark. Maybe I've been lucky but with club type rides and families in that situation, often the first callout or bell ring triggers their leaders / parents to start urging the rest of the group aside, because they at least have the experience to realize in retrospect that blocking the whole path was wrong.