My guess would be shock. When I was robbed I didn't think to attack the guy and in fact wasn't even going to call the cops. I staggered home and started cancelling credit cards because that was all I could think to do after throwing up an angry Facebook status warning people to stay off that path. And there were police phones that I walked past on the way to my place. Literally had no idea what to do.
The guy that tackled the Parisian AK train guy had military training. Extensive attempts to break the thought process and create action. Willful disregard of your self is not innate in all but a few. I think a lot of military guys secretly hope to be there when shit goes down because they know what they would do and how surprising it would be to the other side.
Four people attacked that guy and the first and one other did not have military backgrounds. I do not believe the guys on Flight 93 that attacked the highjackers had any training either. Nor did that child that tackled a suicide bomber outside of his school. So I would think that surely there must have been someone that would react like that. <Insert silly French retreating joke here>