I had to ask a question about this as well, because it's something I haven't REALLY paid much attention to - this was specific to the bow direction change on the current Xmas project.   

It was well explained by FM himself - my "reasoned question" is here - https://fiddlerman.com/forum/c.....-2/#p64501

and Pierre's response is a couple of posts further on - https://fiddlerman.com/forum/c.....-2/#p64522

....  and indeed I was doing much as he suggested, slurring 1/8th notes where no slurring was indicated - BUT - the other point that escaped me was the occurrence of two consecutive up-bows  - and now I understand it ( but still find it hard break my current bowing habit ) -

  The only places he has indicated bow direction in the score extract on these pages are the up-bows on the last note of bars 2, 4 and 7 - which is precisely where the "expected" bow-change-direction is NOT where you might expect it....  hmm like instead of playing D, U, D  - we are instructed to play D, U, U - leading of course to the "expected" Down bow on the first note of the next bar.....

  For an easy tune, it taught me more than I expected!   

...hmmm as to that - well - I'll try to help although I don't know enough about this really - my response was specific to "something I learned from Pierre in response to my question"...  I rather *suspect* that there are differing situations where the "accented" beat, or rhythm, or note, is not necessarily the first note in a bar- and that may be more to do with the style of music played - but I don't know enough about it yet to say really - these are just my thoughts / feelings about it...  nothing more.

From where I stand, I suppose I would look at it like this - in the absence of bowing indications, I would at first assume that the first note in a bar would be a down - but - as I say - it probably depends on the style (and time signature amongst other things) of music being played - I just do not know if that is a "hard and fast" rule - and I rather suspect it is not.  (ready to stand corrected! LOL)

I have an absolutely HORRID habit to break - I accent on an up-bow - it was just what "came naturally to me" simply because I was picking out tunes on the fiddle before I even started to seek advice on playing or joined fiddlerman !   So my bow movements are often "in reverse" to almost everyone else (apart from my unintentional slurring mistakes - which makes it even worse! LOL and TY again for that @Fiddlerman )

But yes to answer your question (finally) - in THIS SPECIFIC CASE in the example given from the FM Xmas score - the indication is that the three 1/4 notes in the bar - which are separate, and not slurred, "suggest" that the bow changes direction on each note - and WOULD be played separately as a D U D  sequence - but - they have to become D U U to get an accented DOWN on the first note of the next bar..... and I guess you have to "read ahead" and notice the indicated "Down" to figure what the score-writer intends you to do.  That is, you are given a very specific indication of a Down-bow on the first note of the next bar....  so you ought to finish the current bar on a up, up - so - hmmm indeed....

(but, hey.... I just accent notes regardless of where they appear on sheet - I just try to do what "kinda sounds right" - so you can tell I'm still working on it and have a LONG way to go! - so I'm definitely not the best person to ask - but it helps to share our problems, for sure....)

Fiddle on !

I've taken the opportunity to markup @Fiddlerman 's score as shown with bar numbers - easier for my analysis ! LOL - here

 OK, to try to answer FM's question ( feels like an exam! LOL ) - 

Bar 17 - the question of FM's added slur - well, we're in the "right" situation to start the bar on a Down bow. 

WITHOUT the slur added by Pierre, we would play (and I'll look ahead) D U - leading to a Down for 3 beats in bar 18 and an Up for 3+2 beats (slurred B across bars 19 and 20)

But with the added slur, we would play - bar 17 Down, across the two slurred notes, U for 3 beats on bar 18, and a Down for the long, sustained, 5 beat slur across 19 and 20.  And, being a down, we're starting from the frog end, there will be greater weight and attack, with the volume (although not indicated) dying off, diminuendo-style - also - it's the end of the singer's sentence - And THAT'S the give-away for the Up indication on the second note of bar 20.    

The added slur in bar 24, leads to the same situation at the very end of the piece - in bars 26 and 27 we've got that same, long, sustained 5-beat slurred "single note" - starting off on the accented down, and tailing off - as it probably would be voiced by a vocalist.

Anyway, that's my answer !  LOL