The most common immediate reactions to the explosion were surprise, joy, and relief. The Manhattan Project
People are different and so express their feelings in many different ways. That's what happened to the people who witnessed the Trinity Test. Because they lived differently the years before, they didn't react the same way when the experience took place.
In this excellent article, "How Oppenheimer weighed the odds of an atomic bomb test ending Earth", Mark Johnson writes:
The nightmarish idea was this: The tremendous fireball generated by the bomb is so great that it heats nitrogen atoms in the atmosphere to the point that their atomic cores, called nuclei, fuse. The energy released causes more nuclei to fuse, triggering a runaway reaction that sets the atmosphere ablaze.
And this is the way the world ends.
Except that it didn’t. And physicists knew it wouldn’t, long before the Trinity test.
But no matter what, they all had pressure. They had worked very hard and had great responsabilities. Physicists, soldiers, family, staff, all who knew about the bomb had different levels of knowledge and very different personalities. Not everyone feels and reacts the same way. Some feel joy, some feel relief, some feel fear... and they express their feelings just when it's done, in their own way. That's just what the movie is showing, almost quoting Oppenheimer's recollection of the events.