A series of flowing lines suggests movement. It is an ideograph for the verb to spill.
Illustrative Text Reference:
Mohenjo-daro: Seal: M-1707 a: Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and Inscriptions: Volume 3,1: Page 23: New material, untraced objects, and
collections outside India and Pakistan: Suomalainen Tiedeakatemia.