I still don't know how that hacker was able to breach my account. I tried to recap what I did these past days and weeks. As an internet guy for a long, basically, I know the basic and advanced safety measures for protecting my personal account. Although there's no 100% guaranteed protection, I'm really sure I didn't do some crappy moves that lead to this. My Bitcointalk account was only accessed here in my IP's home and on my mobile data (single network) when away.
It is more obvious that you made a wrong step somewhere, because if this were not the case, someone else would not have gained access to your account. Regardless of where you store your login information, the moment you enter it in your browser, it becomes vulnerable if you have a keylogger or RAT on your computer or smartphone.
Did you check your IP logs as advised by @tranthidung, do you see the IP address the hacker used?