If Old MacDonald (a new course) is using "sleepers" in some of its bunkers I would have to say the reason that was done was as some kind of tribute to the old days of golf course architecture.
The good news is we do have a few contributors on here who would most definitely know the answer to that!
I believe I can say with pretty much total assurance that the reason Pete Dye began to use railroad ties in his architecture often enough that his style became known for that is because on their rather well known study tour of Scotland many years ago Pete and Alice became just as fascinated by some of the really rudimentary old man-made features of early architecture (such as sleepers) as anything else they found over there.
I believe it can be pretty well documented that it was Pete Dye who rejuvenated the use of "sleepers" over here about a half century AFTER they basically went out of fashion and use over here.
A decent example would be NLGA's famous "Short" hole (#6). Originally it had "sleepers" supporting the bunkering along the front of the green but Macdonald removed them many years ago and went with the look the hole has today.
Personally, I have nothing against the use of "sleepers" in architecture today or again but I do believe it probably is a bit "gimmicky" at this point in the evolution of golf course architecture.
Furthermore, I know if I was an architect and I used them I would definitely tilt them and fairly substantially towards the line of play (as they seem to be in that photo of Old Macdonald). If an architect stands them straight up in a sand bunker there is no way they could not be considered an accident waiting to happen.