Casus belli apparently relates to apparent conditions that leads to another one which is war. “Casus” relates to conditions like a case that leads to another one. Cause, case... “Belli” just means apparent in Turkish. Apparent conditions. As simple as that because it is why they were named as so. Very strict conditions written to avoid misunderstandings.
Conditions constrained to not overlap.
(It is possible that the use of this term relates to the cultural norms of that era in the region, as the waist was the only body part that clothes were adjusted to make apparent. Constrained to not over lap. “İnce belli”: Thin waisted.) (Thin spread :D)