meaning of "They’ll be sorry they sawed off the branch they were sitting on"
The situation is A was annoyed by few people, So she decided to impose a boycott and then complaining about them and said that! It's nothing to do with the branch.

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If you are sitting on a tree branch and you saw it off, you will fall and hit the ground hard. That is the meaning of the phrase.

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it is a proverb type expression.  It means to do something unwise and cause harm to yourself or your situation. Not a very smart decision to cut the branch that you are sitting... If you think of exactly what that person was doing then you can see the corelation between the expression and what that person did.