The prolonged contraction of the Ottoman empire supercharged problems with minorities. As Christian states gained independence and/or new territories there was a tendency for many of these states to expel or kill Muslims from previously mixed areas since some influential persons regarded them as a potential fifth column who may want to help the Ottomans to come back due to shared religion. So for instance Serbia enacted a policy to remove Muslims from the region of Nis after gaining control over it. As Muslims would flee the contracting borders over to where Ottomans still held sway, though, these refugees would be quite angry and vengeful towards local Christians who they would often view as potential traitors who could take their homes just like at their earlier homes, and they would tell awful tales of what happened to them that would radicalize local Muslims. The presence of the refugees would also tilt the ethnic balance towards their favor. This combination of being locally outnumbered by people angry at them could lead to riots and massacres, not a safe situation, so the Christian minorities would often themselves pick up and flee across that same border going the other way… and guess what? These Christian refugees had a very similar chip on their shoulder and reacted in similar ways when settling in areas that still had mixed populations! Once the ball got rolling the process of ethnic polarization could autonomously sustain itself without the governments involved needing to keep on supporting it since it was based on the rage of local people. Sometimes governments would lean into it for reasons like to exert more control over distant restless areas and sometimes governments would pump the brakes for reasons like wanting the taxes of the people locals were wanting to expel. Kind of depended on who would be in charge, the problems they were dealing with and how they prioritized them. Going back to the Serbia and Nis example, Serbia got an ethnically ‘clean’ Serbian Orthodox Nis, but in doing so the Muslim Albanians they kicked out set up in previously mixed Ottoman Kosovo and that combined with the accompanying outflows of Serbians decisively shifted the demographics on the ground in Kosovo to majority Albanian Muslim.

A good chunk of people in what is modern day Turkey, something like a quarter or third, are descended from the millions of Muslim refugees who fled areas like the Balkans and the Caucasus (where Russia was slaughtering Circassians); these refugees are called Muhacirs from an Arabic word for immigrant/emigrant. With so many having these sorts of traumatic experiences or knowing someone who did (and the leadership in Istanbul being exposed to many many many of these people given that Thrace is the area many of the Balkan Muslims went to), the late Ottoman Empire and Turkey developed a very strong sense of paranoia and suspicion towards the remaining Christians in the empire.

If you had to chalk it up to a foreign power I would say Russia was both the main reason for Ottoman control slipping in the Balkans and through its internal policies of expulsion and extermination was the source of over a million Muhacirs which was a big contribution to that process of ethnic polarization. But the French and the British were certainly happy to pile on with divide and rule strategies to inspire revolts when they found themselves on the opposite side of a war with the Ottomans, and to maintain such policies for ruling what they gained.