I'm not entirely sure what happened there. It almost looks like the descending guys twisted the ladder, or tried to 'walk' it while they were on it.
Ladders, btw, are rated for full load on every side that has steps. That is, a 250# rated mechanics ladder (arranged like the one in the video) is rated for 250# on both sides at once.
Looking at the ladder as it lay on the floor, it looks to be in perfect condition. That is, no remaining twists or kinks in it. I don't think the ladder 'failed.' I think this was an 'operator' failure.
I think the problem is that ladder is not built like a step ladder,wider at the bottom.It looks like it was not all that stable.If it had been used like an extension ladder it would have leaning against a wall and been stable.
That is a extension ladder pulled apart and each side used as a rigged step ladder. There is no spreader bar between sides and the rungs show no locking cogs like used on the multi type ladders.Those short cheap extension ladders will flex a lot when together and very flimsy when separated.