And I haven't stored MtGox password into my computer.
Basing on that, I assume that you don't use any password manager. And basing on that, I assume you heavily re-use passwords between websites, as it's impossible to remember hundreds of
really different and
really strong passwords (especially for websites where you log in once a month).
You should consider possibility that some other website was compromised and you used the same email and the same / very similar password there (or you might even sign up to some website which has a fraudulent owner, especially bitcoin-related website).
I followed every possible secure practice except one, not using the double security measurement
Well, there are at least two best security practices which you didn't use and the second one is not using a secure passwords manager with strong encryption, which allow you have very strong, totally random and totally different passwords for each account you register. And since you thought you followed everything but one, there might be some more good practices you omitted as well.
I'm not trying be hard on you, I just throw ideas to consider possibilities. I think if the fault was on the MtGox side, we would read today about unauthorized withdrawals for amounts totaling up to dozen thousands of coins.
not using the double security measurement, which I didn't have a reason to use it at the first place
Well, one could say that it's third best security practice you didn't follow - not using a security layer where it's available (and doesn't come with much disadvantage, e.g. being difficult or uncomfortable, as you can setup 2-factor auth only for withdrawals at MtGox).