So I noticed this irritating verbal tic several times recently in the speech of expert witnesses interviewed on NPR, brainiacs who begin sentence after sentence with an unnecessary "so." The sort of thing that, once noticed, can't be unnoticed.
Figuring I couldn't have been the first person to pick up on this, because I so rarely am, these days, I went Googling; learned that the first known account of this behavior appeared in a 1999 book about Silicon Valley. So it's a subculture usage that went general, adopted by people who want to sound like techies.
This New York Times article on the "so" plague includes a classic example of NPR-speak: "So it’s, I think, the fifth largest in the nation. So, but now that’s the population in general. So there are sort of two, there are two things that are circumstantial.”