Someone is getting into the accounts or something like that because the DB got hacked? I'm not sure. @Philly did cover this in another post. Only 2011 accounts can be messed with, as long as you signed in after 2012 you're good.
As to my knowledge, Philly activated a MyBB plugin that checks for insecure accounts for a specific date range and checks if the accounts are open for vulnerabilities. This is nothing to be afraid of and we should just move along.