Yep, very possible indeed. There are all kinds of ways this could be done. To name a few:
1. You have an easy to guess password and somebody has successfully guessed it.
2. Somebody has seen you entering your password.
3. Malware installed on your machine (or a machine you have used to open your Yahoo account) has transmitted your password to a malicious user.
4. You have used a public PC (an Internet cafe or library, perhaps) with a hardware keylogger on it.
5. Somebody has installed a packet sniffer on a network you have used to access your Yahoo account.
Assuming you are following basic security precautions my guess would be method 1 or 2.
Ed Metcalfe.
Please do not feed the trolls.....