Why was Anne Frank's family hiding place discovered?

If they didn't, Hitler would track them down and separate them from each other and put them each in a concentration camp. They would die.

Margot received a letter from the Nazi's asking for her (as they picked out random people) They had to reply otherwise the Nazis would come looking for them and take them to the concentration camps and starve them. this is why they had to go and hide in the secret annex.

Anne Frank parents had plan to move into the secret annex with was situated in the attic of Otto Frank (Anne's dad) building once Hilter raised to power. However, they moved in weeks before they had planned when Margot Frank got a call-up from Central Office for Jewish Emigration ordering her to report for relocation to a work camp. They moved into the secret annex on 6 July 1942 where they were joined a week later by the van Pels family and later in November by Fritz Pfeffer.