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Why does my bank suddenly need to know where my money comes from?
Bank runs very complex software to detect suspicious activity - terrorism financing, money laundering, etc. How would a program know that some person's activity is suspicious? It uses a set of rules. That set might be imperfect (that likely was not intended) - there might be some rule that triggers a warning on your account dominating the fact you've been with them for 15 years. So it's highly likely that an imperfect program triggered a warning on your account and the bank employer didn't dismiss it.