What would be the reason why someone would need second heart bypass surgery?

Progressive disease. After a first coronary operation, control of risk factors is preventive. Non smoking weight control exercize blood pressure and lipid monitoring and control either other native blood vessels, or the grafts can be involved with progressive atherosclerosis.
CABG. After time the vessels involved in the first bypass may block. There are other blood vessels which weren't involved in the orignial surgery and they could cause problems which would require surgery on them. This is why we want patients to work hard trying to prevent more trouble after an interventon like bypass.