Monguor is one of the offshoots of Mongolian that arose because the Mongols conquered China and ruled it for about a century. Most of China's border garrisons were composed of Mongol-speakers, some of whom took root. For example, there are a few Mongol-speakers near the Vietnamese border, others in northeast Manchuria (later relocated), others in the provinces of Gansu and Qinghai, and some even in northeastern Afghanistan. Their languages have of course diverged over the centuries from the standard Mongolian of the Mongolian Republic and (Chinese) Inner Mongolia.
The speakers of Monguor in Qinghai have been living with (Han) Chinese and especially Tibetans for a long time. Unlike any other Altaic language, their language has developed consonant clusters at the beginning of syllables. Well, nothing strange about that: some short vowels fell, and the result was a bunch of clusters: it's happened in lots of languages, including English (more at the end of syllables, though, as in /lɪvd/ < /lɪvəd/ 'lived').
But just which consonant clusters arose and which didn't? The Middle Mongolian verb stem [hudaru-] 'destroy' became [xtaru-] in earlier Monguor, and [stari-] in the current language. But [hulaːn] 'red' did not become *[xlaːn]. (In standard Mongolian, it has become [ulaːn], as in Ulaanbaatar 'Red Victory', the capital of Mongolia.) What made [xt] privileged and [xl] not?
Well, most Monguor have for a very long time been bilingual in Tibetan, or the local variety thereof, in which [xt] is a valid cluster but [xl] is not. So when the Monguor borrowed the Tibetan word [xtorma] 'sacrificial offering', they did not adapt it to their Mongolian habits, but took it in directly as [xtorma]. Having learned to pronounce [xt], they adapted some of their own words to use it as well. In fact, vowels fell if and only if the resulting consonant cluster occurred in Tibetan as well.
How is that for high?
Primary source: Rona-Taš, A. "Remarks on the phonology of the Monguor language", Acta Orientalia Academiae Scientarum Hungaricae 10.3 (1960), pp. 263-67. A more accessible source is Robert Ramsey, The Languages of China.