How does the atmosphere in the house become terribly strained by one incident?
The atmosphere in the secret annex is constantly strained by the simple fact of so many people living in such close quarters under fear of discovery, deportment, and death.
There are two crisis moments in the play before the denouement and resolution. First, during the Hannukkah celebration, an intruder in the office below hears the crash when Peter falls off the chair while trying to turn off a light. The inhabitants of the annex now know that someone knows they are hiding.
The second crisis occurs when Mrs. Frank catches Mr. Van Daan stealing food. She demands that Mr. Frank throw Mr. Van Daan out. This crisis is mitigated when Miep arrives with the good news of the Allied invasion.