The triggering events was not made by many single individuals, which seems to be your underlying assumption. There were groups of people who decided to join, this makes the case different. In cases were individuals do not interfere when common logic say that they should, I believe the case would be different if a group of 5 close friends were expected to interfere by common logic. Our groups are our shields, and we feel and act different being around a bunch of people we know well. A group being at least more than three individuals.

A group of 10 friends are sitting in a circle, drunk, high or whatever, they’ve been paying attention to the guy dancing for some time and some of them may say “hey we should just run down there and join”. So imagine if there were many of these groups paying attention, you would just need some few people in the group with influence to say “yeaah let’s go!, cmon cmon!”.
Now if there were just a bunch of singles and duos the effect we saw probably wouldn’t have occured. But in this case people act different when they are around a big group of close friends.

My point here is that this can happen because GROUPS of friends decided to run in.

There are many individuals and duos that dance with him through the video, but it’s finally when a big group of people come running down that the mania starts. This group of people were probably a bunch of friends sitting together eager to dance, and I can imagine the situation “cmon let’s go dance!” by 2-3 of them, finally all of them are in and they run down TOGETHER.

How do you think generals led armies clashing straight into each other in arms combat in former days? Humans act different when they feel like a part of a unity, while you are refering to an effect happening when people are not part of a unity. This is logical, humans are pack animals and therefore we withdraw when there is a sense of uncertainty. In an unexpected scenario we don’t play sherlock holmes if we’re alone.
At least the majority of us.