Use 'contributory' to describe something that helps cause another thing, but isn't the only reason. Poor diet is only a contributory cause of that health issue.
Pro Tip 2/3
Often Negative
It's commonly used in negative contexts, like problems or diseases. High stress levels are a contributory factor to heart disease.
Pro Tip 3/3
Not Sole Cause
Implies that other factors are involved beyond the contributory one. Lack of exercise may be a contributory cause, alongside genetics.