Maybe you used some interenet cyber cafe that had computers infected with keylogger or maybe you logged in to your email from an unsecure network and you did not use https connection. If someone was sniffing packets they would get your email account and then based on what you have in it they go from there…
You may have also opened a phishing email too and logged in thinking it was moneybooker/skrill
guessing password is the least likely unless you have something silly like abcd etc