This is a typological study of the why-interrogative expressions in Tibeto-Burman languages, or
the non-Chinese Sino-Tibetan languages, in Sichuan, southwest China. After examining 22 languages/dialects, the interrogative
category of reason is grouped into three types, viz. the distinct why-type, the how-type, and
the what-type, according to the interrogative categories from which they are derived. It is discovered that the
what-type is the dominant strategy for languages in Sichuan to derive the reason category. It is also common
for Tibeto-Burman languages to use verbal interrogatives, namely interrogative verb phrases and interrogative words with verbal
origins, dominantly in the sense of ‘to do what’ and ‘to become what’, to ask for reason. After a focused investigation of the
verbal interrogatives, it is found that Tibeto-Burman languages in Sichuan distinguish purpose from cause via word/phrase
distinction, sentential positions and semantic differences. A path of grammaticalization of the verbal interrogatives is thus
proposed and analyzed. Finally, the derivation maps of interrogatives in Cysouw (2005)
and Hölzl (2018) are complemented with a refined distinction between purpose and
cause.