This is a nice example, Hive account@lifenbeauty. We are social animals after all, and want to be like the others to be accepted, so if we are the first person from the crowd to do something different and to help, we might be seen as 'different', 'awkward', like 'look at her what she's doing, is she crazy?!'. And that is what prevents people from helping in such situations. If any other people would be helping, most probably most of the crowd would try to get involved. There were also experiments with one person helping, and looking how many people would help if they see someone else helping - but I forgot the outcomes, I studied if a few years ago :D!

It has also been shown how the aggression can be disinhibited with the diffusion of responsibility. Like in this experiment: https://thesituationist.wordpress.com/2011/01/12/the-power-of-suggestion/