it is one of the many cases of loss of many cases that occur. The average cause is a hacked private key, so we must be careful and careful to backup our private key. My advice, keep private-key in the offline network, because it minimizes the incident hack, because most hackers hack through the Internet network.
Yes one of the best way is to keep your private key offline and make sure that your computer is free of virus, hackers will do everything to attack
your system so make sure that you have every precautions to avoid them and became a victim.