Just caught up on this thread... and to the OP @stringy - yup - I know what you mean - you've brought up an interesting point....

and @AndrewH and @ABitRusty and @Gordon Shumway 

AndrewH said
My guess is it's mostly a matter of attention being directed somewhere else. Even after playing for 20 years, my bow occasionally strays onto the fingerboard or hits other strings when starting to learn a difficult piece. There are just so many things to think about in playing the violin, and it simply takes time and repetition to make them near-automatic.

About sitting: ideally you should aim to hold your upper body exactly the same way when seated as you do when standing. That isn't always possible in an orchestra setting (sometimes the stage is too crowded), but at least until you're more comfortable playing, the adjustment is easiest if you're sitting on the edge of your chair, as if about to stand up. 

Absolutely, all of you.  Yeah I get that same issue - usually when playing live, even in front of a small audience, be it friends or on an SJ session.  

What gets me is not so much "nerves" - definitely not that - I mean my playing is for fun and entertainment, nothing more - but - it is a "changed situation" and of course you naturally "want to do your best" - I mean to say, it's soloing - not even a backing track, and any little excursions from the "plan" ( like an individual interpretation of some well known piece) - simply "has to work" to make it recognisable - slightly-off intonation won't matter over much (really) - wrong notes, hesitations - well - you just have to go with it and accept it happened. 

Posture is always an issue for me, and something I have to pay attention to - I normally play standing.  When sitting, yes, I am very aware of the changed "bowing-environment" - and indeed - "sitting forward on the chair/seat etc" and "upright" - brings things back to a playing normality (for me).  When faced with the camera (and in my happiest playing-posture - standing) - it matters not whether I am playing live on SJ or just for a local recording - the same thing I mentioned earlier immediately happens - it's a solo, it's being (or going to be) listened to - and the things which were once automatic (with clean playthroughs during practice) suddenly become a conscious moment - like - "oh, you are sometimes a tad flat on that fourth-finger" and the thought - GET THAT NOTE RIGHT this time - kind of thing - creeps into my head (and affects playing)

In these situations, yes, because of the "creeping thoughts" - sure - my bow can wander, and, worse still, I'm trying to accommodate that (once I'm aware) and it also leads to an improvised bow-direction change when I wouldn't normally do so.

It's (for me) all down to not so much a "lack of concentration" - it is almost like being "unnecessarily overly focussed" on what I'm doing in the moment.  I NEVER play from sheet (although, naturally, I use sheet a lot for practice and learning new-to-me pieces) - when I play, it is stuff for entertainment, fun, traditional, folk, etc music and always an intentional "interpretation" of what may be well-known pieces.

Just a final thought @stringy, having "talked that through with myself above" - is it possible that when your "bowing goes all over the place" - could it be due to over-familiarity with certain pieces ?  What I'm getting at is - I'll bet you have a good few tunes "under your belt" that you can play blindfold.  You have reached that goal (for these tunes, that is) and just assume you can can play with the same ease and freedom of expression on any OTHER ( less played ) tune ?  Honestly - I don't know - but I HAVE experienced that early on...   NAILED a few tunes.  Played them (literally eyes closed) perfectly (for my requirements)...  Thinking I can do the same on the next (less much played) tune - and NOOOOO... I'm all over the place as well.

Dunno, stringy - just thoughts mate -