platform, unit, term of address
坮 centers on a raised flat surface used as a base or stage: a platform or unit. The term-of-address sense is a rare extension, likely from the idea of a formal standing place.
坮 combines 土 (earth) with 台, which likely contributes the sound and the idea of a raised platform or base. The exact historical development is uncertain.
The left side 土 is earth, and the right side 台 looks like a raised platform. Picture a platform made of packed earth: 坮 is a platform.
For タイ, imagine a tie (ネクタイ) draped over the earthen platform: tie -> タイ, and the platform holds the tie.