In Chapter 12 of The Chrysalids, what causes the group's flight?
When Anne commits suicide, the others find out about the telepaths and they have to flee.
Anne’s note after she commits suicide gets the telepaths worried that they have been discovered.
Michael seems to be the one who is most nervous about being found out.
The discussion, and Michael's advice, made the threat of discovery seem both more real and more imminent than it had been when I talked to Uncle Axel earlier in the evening. (ch 12)
Michael even makes preparations so that he is ready for anything. Three hours after he falls asleep, he gets the warning that they have started being taken. Rosalind uses her mind to reach David.
'David,' she was telling me, 'we must get away at once -- just as soon as you can. They've taken Sally and Katherine -- ' (ch 12)
They have started raiding the telepaths’ homes in the night, though they are not sure how many there are or who they are.
This is when the telepaths flee to the Fringes.
Anne's marriage is the beginning of the end for David and his kind. She marries a normal person, and then tells him about her ability. When he does not accept it, Uncle Axel kills her new husband to protect the telepaths. Anne asummes one of the telepaths is responsible, so she kills herself and leaves a note implicating them and naming all of them--even little Petra.
Anne's predicament is an example of how untenable the telepaths' situation is. They cannot marry normal people who might betray them. They live in fear that someone will find out who they are.