Why did Eastern Cham and Tsat develop tone systems?

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Austronesian languages Major divisions of the Austronesian languages.

Eastern Cham and Tsat developed tone systems primarily due to prolonged language contact and bilingualism with neighboring tonal languages.1

Specifically, Eastern Cham acquired these features through its contact with Vietnamese, while Tsat developed them as a result of its interaction with both Chinese and Tai-Kadai languages on Hainan Island. This process of areal adaptation led these two Chamic languages to become largely monosyllabic and tonal, distinguishing them from most other Austronesian languages.1