We all know bitcointalk was hacked in 2015 there is a lot hacked accounts. May be some backdoor in forum software, they are using Powered by SMF 1.1.19 | SMF © 2006-2009, Simple Machines software, which is very old but that doesn't need to be reason. I'm on that forum from 2013 and got hacked in September this year. I don't know how that happen.