When an attacker gains access to your email account.

If your password was not strong enough then the attacker might have simply guessed your password through trial and error. It is common practice these days for attackers to use sophisticated software to guess passwords for online accounts.

Another strong possibility is that your computer has been infected (0wn3d). If an attacker is able to gain access to your computer then it is very likely they have access to all of the passwords on your computer as well as any that you have typed in manually.

In any case, if you suspect that your online accounts have been compromised you should check your computer carefully for any sign of viruses, trojans, root-kits, key-loggers, or other malware. Be sure to do this before changing your passwords and/or change your passwords through some other secure system. Otherwise the attacker will know all of your new passwords also.

Further Reading:

Handling Compromised Email Accounts