I think it's another bug.
You are not 'locked out', blocked, being made inactive.
That member account number had a rating other than member on it (higher) but that rating was not reflected on the admin pages which show those rated people. Which was very strange. So quite how the account was raised is beyond me, as is why it suddenly locked itself.
There have been two other bugs recently to do with threads closing themselves so I'm guessing this is a third bug. I know what you have posted about and why you might think something is up with this, but from where I am all I can honestly put it down to is a bug.