Hard to say without seeing the gate from the viewpoint of the riders but probably something relating to the background either being distracting or there being little contrast between it and the barrier. Maybe it's the opposite and there's something about it that encourages "target fixation" where if some hazard gets your attention too strongly you'll stare at it and ultimately collide with it, potholes for example. Then there's the straight run at it maybe giving rise to the phenomenon of "looming" where an object approaching from a distance doesn't get the attention that something moving across your path does, until it's suddenly too close or too large to take evasive action or brake in time. Or, as previously suggested it might usually be open and for some reason it's now left closed.
Despite all these suggestions there's still no substitute for looking where you're blooming going. No point in bleating about it not being there last time when you can manage to avoid all those dogs, people, cars, etc which are always being there or not-there in a random manner.
Although I can empathise with the pain those riders must have felt, I must be a terrible person as I enjoyed the schadenfreude of seeing them doing it to themselves as they were going too fast to stop in time. I just hope none of them were seriously injured. I was also distracted by the other videos which inevitably linked to that one. Some were awful, some were genuinely funny. I suppose everybody enjoys a pratfall, if it's happening to someone else.
