I'd suspect a lot of forum "hacks" are due to more important databases that were cracked, and people reusing login credentials. IOW, a lot of people will use their yahoo login for everything. Search a username, then use the same password from yahoo. Good chances of it working, especially in years past. Hopefully people are getting smarter about it, but I wouldn't put money on it.
This. Threat actors not only use brute force attacks but they compile (and sell) databases/dictionaries of leaked and usernames and passwords. So that old Yahoo, Experian, facebook, etc. breach has helped create lists of valid (at one time) password, usernames, or cominations of both that are easily used in attacks. So instead of guessing, the bad guys just repeatedly apply that dictionary to websites to try to gain access to accounts.
Want to know what service from "not-a-security-company-but-we-offer-marginal-security-solutions" allows unlimited log-in attempts by default? It rhymes with "poffice365". The default is unlimited login attempts without account lockout. Bad guys can spend infinite hours throwing dictionaries and random garbage at those accounts. Since so many people use their corporate email for personal things AND recycle passwords between sites, we see compromised account in O365 ALL.THE.TIME.
And congratulations on helping add valid passwords to a list of valid passwords that the bad people use to access accounts. If I were a bad guy, this is probably a nice list of passwords to add to the bucket. Now they only have to apply some of the best passwords to all their attempts.
IP address helps since it gives them geo-location so it isn't trivial. So they now know what country/state you are in and your password. Perhaps there are lists of people, their email address and where they live in that Facebook breach? Maybe you re-use your username or parts of it for your on-line banking? Maybe you have re-used that password you just entered into my cool "check your password" site or maybe that site gets breached? Cross referencing that password to my know list could yield some data. Just speculating...
I also am waiting to see someone apply some true analytics to something like the list of passwords to see if there are any subtle indicators on how people choose passwords. That could be an evolutionary step in cracking passwords. Or I could just email you and tell you your credit card bill is due or your mailbox is at quota and link you to a fake site. A lot of people give up the goods on this one. Waaaaay easier than trying to crack a password. I can't spend time hacking. I'm busy. I'm a goddam Nigerian prince with money to give away.