Airlines allowing queuing on stairs?

Just curious on thoughts regarding this as we travel in the airport this morning. We were discussing how airlines - primarily Ryanair, from experience - regularly have passengers queueing on stair passageways, sometimes for up to 15 minutes at a time until an aircraft is ready. Is that actually legal? We were discussing this today and how there are no other situations/public spaces where that would be allowed for health and safety. Could something going wrong potentially lead to lawsuits and/or investigations?