Originally the wooden frame that held ritual bells and chimes: heavy, serious apparatus. Grand undertakings borrowed the image — enterprise, profession, one's life work.
毕业 to graduate ('finish one's studies'), 工业 industry, 职业 profession, 作业 homework — the character follows you from school assignments to career. Buddhism uses 业 for karma: your deeds as your work.
A rack of BELLS to maintain — serious WORK, a PROFESSION.
作业 'homework' and 事业 'career' share 业 — school trains you for the same character you'll work under forever.