All hard disks develop bad sectors in use. As hard disks age bad sectors will appear more often. Magnetic media is not 100% reliable in every spot across the surface of a hard drive platter--this is a simple fact of data storage engineering.
Bad sectors are areas on the hard drive platters that either cannot be read from reliably (data read from the track the first time does not match the data read from the track at a later time), or cannot be read from at all.
You are not usually aware of this because hard disk drives have automatic error management built-into their firmware code. The bad sector cannot be moved from one spot on the hard drive platter to another, but the error sector can be marked to prevent its use, and the address of a "spare sector" on the drive can be substituted in place of the error sector.
This usually is a spot on the hard drive platter that is not adjacent to the error sector, so the heads have to move to a physically separate location to retrieve the data that is supposed to be in the error sector. Absent fragmentation effects, this is why hard disk drives tend to "slow down" as they age.
Once you start becoming aware of bad sectors, in normal use, the spare sector pool has been exhausted, and the sectors that are being automatically marked as bad cannot be re-mapped. From your point of view, the drive is "suddenly" producing bad sectors when, in actuality, it has been finding and hiding bad sectors the entire time. The difference is that the firmware remapping routines can no longer make the process hidden, and you start "losing" storage space to the new bad sectors cropping-up.
At that point of awareness, it's usually a very good idea to replace the drive before you lose data, or it becomes unbootable, and you lose the entire OS installation.