There were several Hazara uprisings against the Afghan government in the 1880s and 1890s, but they were all crushed pretty quickly.
How about this: We tweak the treaty at the end of the Second Anglo-Afghan War in some way that really angers the Afghan government. There are plenty of ways to do that. The British could demand more territorial concessions or they could refuse the Afghan demand that they staff their mission in Kabul entirely with Indian Muslims instead of Europeans. However it happens, the Afghans are enraged and instead of turning inward and focusing on re-establishing stability, they start cozying up to Russia in hopes of getting out from under the British yoke. From there, Great Game politics take over and the British support one of the Hazara uprisings with the goal of creating an independent Hazarastan as a counter-balance to the increasingly pro-Russian Afghans. Hazarastan is initially a British protectorate, but gets full independence in the mid-20th Century during de-colonization. Does that sound plausible?