They were laughing at the later part of the scene, once it gets tense. I don't want to be inaccurate, but let's say they might have been laughing around the point in the scene when shooting gets started, not (only ?) earlier like when the giant pipe appears.
I don't understand it either, but this clearly put off my GF from any QT movie since.
Discuss films and filmmakers of the 20th century (and even a little of the 19th century). Threads may contain spoilers.