The IRS links this trend to US tax policy, which taxes based off of nationality rather than citizenship. Americans living abroad may renounce their citizenship to avoid being taxed twice over. This caused the UK’s Foreign Secretary Boris Johnson to renounce his US citizenship in Feb 2017.
Some would like to link the trend to the election of Donald Trump, particularly due to the large amount of Americans who threatened to flee the country if he was elected. But the trend began in 2011, while Trump was still hosting The Apprentice.