It all started in a few years earlier. Google had determined that there was a strong correlation between understanding fundamental computer science and being a strong software engineer, so they began testing for computer science fundamentals. At the time lots of companies were trying to mimic Google's success and it quickly became an industry standard. People began just studying how to answer these questions. The population of people that are good at these types of interviews shifted from hardcore detail oriented programmers with academic backgrounds to mostly people that spend time practicing leetcode style questions.