FIFY
How do you know who is security and who isn't? If they are doing their job right, you don't. My guess the non-uniformed security was there and was following protocol. The plan was to contain, but not intervene, but then a third-party guest took action and they had to jump in. Think about it; if the undercover guys had jumped in right away and failed to break it up, this would have looked a lot less like a family squabble and a lot more like a full-blown melee.
How do I know there were undercover security cast members there? Have you seen even one first person account from a guest that stepped in to break it up? How about even one story about Disney thanking the guests for stepping in to help? Mark my words; nearly everyone involved in quelling this was a CM in some capacity or another. If anyone else intervened, Disney did not appreciate it.