In brief :
Free Email accounts get hacked easily via known exploits / or people let free email lapse and scammer renews it .
Keylogger is also an old method ( internet cafe's are notorious )
Email is the "key" to domain ownership ... so scammer has effectivly control your Registrar account and in turn your domains.
In short
NEVER use free email..... if you can afford domains , then you can afford decent email hosting .
use a different ( private )email addy @ your registrars account that the one visible in your whois .
Use
Roboform / roboform-to-go to ensure that you have secure passwords , also aids in avoiding keyloggers etc ..
Dont visit warez sites !
Use good antivirus/anti-trojan.
And when buying domains ... do due dilligence ! If it's too good to be true .... then it usually is .
Dont trust whois 100% ... if email has been exploited already , then the scammer can reply as if he is the real owner. Put a bit of effort in , and use domaintools