Lemons do not need any reason to die early, they just die whenever they feel like it and are likely caused by a material or manufacturing defect.
On the side of age-related failures, one of the most common failure modes is head crashes: a mechanical event or defect causes the head to touch the platters, scrape a bit of magnetic material off the platter, possibly slightly mess up one or more drive heads, the damaged sectors may end up "reallocated", rinse and repeat until the recording layer or heads are too badly damaged to remain usable.