1) user noob atitude (trust an unknown user to sell bitcoin for paypal ... and chargeback begin !)
2) computer compromise (by virus ... but, in usual way, by other person use the same computer and destroy it)
3) user HEAVY noob strategy : trust the hard drive that it hold bitcoin-QT and not backup the wallet.dat
4) user VERY HEAVY noob reaction : trust "virtual" wallet host in the plateform that they exchange BTC because fee to retrieve BTC in bitcoin-QT are too high.
so :
- use bitcoin-QT
- use "one shot" stratégy on plateform exchange (insert USD/Euro, buy BTC, retrieve immediatly BTC to bitcoin-QT)
- backup wallet.dat on 3 different USB storage
- secure PC (like normal way... use a "commercial" antivirus and not a free antivirus)