The number 1 reason people lose data is due to someone deleting it or corrupting it (e.g. by oversaving new contents) The more people that can access the data, the greater the risk.
Hard Drives often spin at 7200rpm, thats 90 times a second, they wear out slowly, can get knocked, zapped by power failures or dropped. Writable CDs and DVDs can get scratched or slowly degrade over time (max 5 years life). Memory sticks get lost, snapped or wear out with lots of use. Backup tapes stretch or wear away.
Closely related (most hacking is done by introducing viruses or trojan horse programs). These can directly affect data, or result in data loss when users or admins react to the infection/intrusion.
You might just lose the device holding your data (e.g. laptops or memory sticks) or someone might steal it.
These kill PCs sometimes, if a disk drive is on when they occur there is a very high chance of some or complete data loss.
We sell a number of backup solutions and can help with most scenarios, just contact us.