'casting' (and thus 'castings') ultimately originates from Old Norse, specifically the verb 'kasta', meaning 'to throw' (through Old English and Middle English the sense broadened).

'kasta' (Old Norse) → Old English/Old Norse-influenced Middle English 'casten'/'cast' → Middle English 'casting' (gerund/noun) → modern English 'casting' and plural 'castings'.

Initially it was linked to the idea of 'throwing' or 'casting' something; over time a specialized sense developed for 'pouring material into a mold' and also for 'selecting actors', producing the modern senses.