As a US citizen in the UK for the summer, it's scary to imagine what I'll be coming back to in the US if the right thinks that the left has attempted to assassinate Trump. Will there be retribution?
Having glanced at the previous comments, and acknowledging that all we're doing is wildly speculating, it is reasonable to assume the crowd was freezing in response to the sound of gunfire. It is a bit absurd to say the crowd was unbothered by the prospect of possibly being shot at close range. However, it is reasonable to argue that gun violence is a normal occurrence in America.
I can somewhat speak to the regularity of gun violence as a former director of programming at a non-profit. Unfortunately, in the ghetto, gun violence has always been present and is not a new phenomenon. I witnessed two shootings near the children we worked with on the street of this non-profit in just a year's span before I left for law school.
I also remember one of my coworkers laughing in awe over how one time a parent of one of our children had sat out on a nearby doorstep to watch two people shoot at one another from a distance.
Now with the spike in random gun violence, even children in middle class and affluent area are typically trained on how to react to shootings
Gun violence has become almost cartoonishly common and Americans do sometimes treat it with more nonchalance than they should (for a historical analogy, look at Vera Brittain's book where she describes how some British people would go out on their roofs to watch the bombs drop during WWI).