It's all the rage, starting sentences with the word So.
I see Radio 4's Today programme covered it back in 2011, noting it seems particularly common among scientist types. Other than that, they were at a loss about its origins.
So. I've got two kind-of theories about this bizarre trend entirely plucked out of thin air. One about the reason people started to do it and the other about where it started.
I think people often do it in a bid to convey authority, to take the senior role in a conversation. Maybe this explains the scientists thing and it'd also fit in with the fact I've noticed politicians doing it quite a lot (I'm pretty sure I noticed Grant Shapps doing it in a recent Question Time episode - the one featuring that memorable Mehdi Hasan Daily Mail bit). I'd be interested to see if other types of salespeople aside from politicians use it too.
Just in that one word at the start of your sentence you're showing the person you're speaking to that you mean business; that you've taken firm hold of the conversational baton; that you're not merely having a chat with them but informing them. So, listen to me, this is the important bit, kind of thing. A shorter version of the old politician's favourite "The fact of the matter is…" (which is usually followed by a non-fact).
I wonder if it started on social media, where it might make more sense to use the word So at the start of tweets, blogposts and so on to make your writing seem informal and most importantly to give it a feel of continuity. A shorter way of saying "Following on from my last post yesterday…". Maybe it's served a purpose in the 24-hour connected thing, of giving the impression you're constantly in touch with your online friends rather than giving intermittent broadcasts, and it's spread from there to speech. And now it's there people are doing it unconsciously.
I also have an inkling it's the offspring of the Australian inflection: by that I mean people who do one are likely to do the other. In my limited experience it seems to be used more in the south-east of England and by a professional/aspirational kind of demographic, though I may be wrong.