The vector would be that your email adress, as everyone elses, became known from the MtGox hacking incident. Somehow someone decided it was worth trying to hack your emailaccount - making it possible to do a pw reset on MtGox.

Why someone targetted you in particular, and managed to guess your email password, is another question. The most likely cause would be that you signed up somewhere else Bitcoin related and re-used the same pw as you use with your email provider.

(If your email account has been hacked you of course need to make sure to clean your account from any hidden forwards and then basically consider EVERYTHING you've ever signed up for as "broken" since the attacker could've used it to gain entry to a lot more than just MtGox)

Of course, all the above is based on MagicalTux both being honest about what the logs say and that if the logs say pw reset through email that there's no way to fool the system into doing pw resets some other way.