Why is everyone startled when the buzzer rings?

Anne Frank's posthumously published memoir about her family's hiding from the Nazis, The Diary of a Young Girl, is heavily-laden with foreboding. Readers of Frank’s diary know the outcome; that knowledge does not detract for the narrative. On the contrary, knowledge that Anne and most of her family and friends would perish in German concentration camps lends the text a particularly despondent tone. Readers know that the family’s hiding space will be discovered and that they will be marched off to certain death. This knowledge is precisely what contributes to the sense of foreboding that accompanies otherwise minute details. Chief among these details is the doorbell. Early in Frank's diary, she describes the disappointment surrounding her male friend’s visit to her home. Answering the doorbell, Anne is confronted by Hello, the young man, who informs her that his grandmother does not approve of their relationship. This early, seemingly innocuous event, in retrospect, serves as a warning that the ringing of the bell will almost certainly portend sadness.
The significance of the doorbell increases in direct proportion to the level of danger in which the Franks and other Jewish families soon find themselves. Aware of the impending threat from the Germans and from Germany’s Dutch sympathizers, the ringing of bell is subsequently met with great trepidation. A knock on the door, or the ringing of the bell, could signal that the Germans have come to take them away. Note, for example, the following passage from Anne’s diary entry dated "July 8, 1942":

Suddenly the doorbell rang again. "That's Hello," I said.
"Don't open the door!" exclaimed Margot to stop me. But it wasn't necessary, since we heard Mother and Mr. van Daan downstairs talking to Hello, and then the two of them came inside and shut the door behind them. Every time the bell rang, either Margot or I had to tiptoe downstairs to see if it was Father, and we didn't let anyone else in.

The sound of the doorbell was hardly the only otherwise routine part of everyday life about which those in hiding grew to fear. Strange footsteps and voices were a constant source of terror to Anne and the others. The doorbell, however, resonated in a particularly nefarious way, evident in the following quote from the diary entry for April 8, 1943:

I'm currently in the middle of a depression. I couldn't really tell you what set it off, but I think it stems from my cowardice, which confronts me at every turn. This evening, when Bep was still here, the doorbell rang long and loud. I instantly turned white, my stomach churned, and my heart beat wildly—and all because I was afraid.

The family in The Diary of a Young Girl is startled by the buzzer because the doorbell represents the moment when their fate is doomed.