The TAX law causing Expats to renounce, is FATCA, the worst law most Americans know nothing about.
The negative impact of FATCA is on middle class Expats too. No other Country in the world treats its emigrants the way American does.
You probably will have to go to Google, to read about it. Most U.S. media have ignored it since 2010 when it was an amendment to the Hire Act.
Google this Headline. Interview: James Jatras, Lawyer and Anti-FATCA Lobbyist
That will explain it to you.
This is also a story of tax complexity and penalties only partially told here. It has come down to a basic cost vs benefit analysis. Can I afford that U.S. Citizenship Tax and Penalty Club any longer? Sadly, the answer for many is NO!