How, in the world, did the US get the Swiss banks

In 2009, UBS turned over information of 5000 American clients. How did the US get them to do it? First, the cross-border practice of the Swiss bank was illegal. Second, they lured client's money for the purpose to evade tax. Brad Birkenfeld, an employee of the bank, was the whistleblower. However, he got 3+ years of jail time. Upon release, the IRS rewarded him over $100 million dollars.