I don't even think they were extremists among the Uighurs or militant groups at all, I think it's an excuse the chinese government made to just suppress them because you know the naive foul idea of others in Europe and US "Islam = Radicalisation = Terrorism" so China takes it as a reason to take actions against the Muslim minority but the real reasons are 2 things I reas about :

1.The region itself of the Uyghurs is full of possibilities for investment because of the rich resources it has but the CCP sees the Uyghurs as a barrier for profiting from this land as they live on it so they take those actions against them for their economical growth and powerso they ensure full control over that land.

1. The CCP is built on totalitarian communism (even though they don't practice communism at all irl) and so one of its goals is to establish atheism and fight religions but then the only religious group that is being attacked are muslims therefore stating that the CCP have a problem not with religions mainly but with Muslims because they were one of the most groups not accepting the CCP especially the Uyghurs who were known for their resistance and separatist movements while the Hui Muslims were treated normally by the CCP for most of history.

I think because reason 1 and the fact that Muslims in china give a great value for their faith much greater than the ccp, the government tries eventually "re educating" them. A typical example of totalitarianism and that violence can come from anything not only religion but ideologies and even atheism itself as long as those beliefs/ideas are being misused totally and instrumentalized