Data source: travel.state.gov
Some countries are Visa Waiver Program participants whose citizens can travel to the US without a visa for stays of 90 days or less. In the B-1, 2 visa chart, South Korea and Taiwan have significant drops because they joined the program in 2008 and 2012 respectively.
This is a weekend project by Nobu. When US Immigration asks Nobu "what is the purpose of visit", he answers he is a software engineer in San Francisco. Email or tweet him for feedback.