Anand Giridharadas of the New York Times traces the origins of using the word "so" to start sentences, and its widespread adoption.
So, it is widely believed that the recent ascendancy of "so" began in Silicon Valley. The journalist Michael Lewis picked it up when researching his 1999 book The New New Thing: "When a computer programmer answers a question," he wrote, "he often begins with the word 'so.'" Microsoft employees have long argued that the "so" boom began with them.
This logical tinge to "so" has followed it out of software. Starting a sentence with "so" uses the whiff of logic to relay authority. Where "well" vacillates, "so" declaims.