|I and also tell me the causes, prevention and introduction of bad clusters|
Old age. It happens to every hard disk eventually unless something else happens first.
No way to prevent it any more than you can prevent death. Everyone is going to die eventually, so are hard disks.
That is why people who know about these things always do backups. It can happen suddenly, or it can happen slowly, but it will happen.
De-fragmentation will do nothing to solve the problem at all, might even make it worse. Might just push the disk over the edge into oblivion. So until you have saved your data, forget about de-fragging