The available data we have are 1)some historical accounts, although no proper modern bibliography exists for Sandzak b)anthroponymy and of course c)limited Y-DNA research in the last decade.
1)The shift from Albanian to Serbo-Croatian in southern Sandzak is actually a modern phenomenon that started in the 20th century and expanded rapidly in the post--WWII era. Furthern north, up to WWII, there were bilingual areas up to Bijelo Polje. For example, Jusuf Mehonjić/Mehoniqi, a well-known rebel from Bijelo Polje was from a bilingual background and after incorporation of the area into Yugoslavia, he eventually settled in Albania.
2)In terms of anthroponymy, I make this distinction between a northern and southern Sandzak because of the radically different anthroponymy between southern, central & northern Sandzak. I've uploaded (from forebears io) the most common surnames from Tutin (southern Sandzak, 94% Bosniak/Muslim) , Petnjica (central Sandzak, 95% Bosniak/Muslim) and Prijepolje (northern Sandzak, 45% Bosniak/Muslim).
In Tutin, most surnames link back to an Albanian fis/tribe (Pepić the most common surname in Tutin- Pepaj until WWII - brotherhood is from Kelmendi, on the other side of the border in Kosovo the same brotherhood continues as Pepaj). Petnjica - just 45 miles (75 km) to the west of Tutin - is very different. There, surnames - in a 95% Muslim area - clearly show that many of these people were Orthodox at some point. We also have some surnames that indicate an origin from Albanian fis/tribes (Gašić < Gashi). In Prijepolje, the situation is even more different.
There we find no surnames influenced by Albanian fis, but we find many surnames influenced by Montenegrin pleme/tribes (Rovčanin, Drobnjak).
petnjica.pngPrijepolje.pngTutin.png
Therefore, I argue that because of a different substratum in sub-regions of Sandzak, different populations tended to migrate to different areas. Kelmendi, Hoti etc. brotherhoods settled in areas like Tutin because they found themselves in a much more familiar environment there and Drobnjak, Rovčan etc. brotherhoods settled in areas like Prijepolje for exactly the same reasons.