Is it possible that the noise from the fireworks triggered a "post traumatic" flashback and put her in "fight or flight" mode? It can be the most innocuous thing that triggers these reactions and to someone on the outside of these experiences, it certainly seems inappropriate and out of place/out of character for the person, and bewildering like this situation was to the people who don't know your friend or her history.
Nothing wrong with taking her aside and getting her re-focussed on the present reality. You're a good friend to want to help her. I somehow doubt that she wants to "rain on everyone's parade" as it were, but perhaps part of her gets uneasy and nervous about having a good time or enjoying herself too much. Survivors of domestic abuse sometime subconsciously come to expect something bad to happen to them just as soon as things seem calm and fun and "normal" again, because that is part of the cycle of abuse. She might need some reassurance that her friends are just that: her friends, and that you have no intention of harming her in any way like her ex did.