I can see memory issues causing (memory dumps) blocking logins. I haven't seen it actually change a login to be expired. I guess it's not much of a reach.
The next time it happens (hopefully it doesn't happen again), you might want to also check the sys.dm_os_ring_buffers for the logon failures. You might be able to procure a bit more information.
And since you saw the memory dumps, what is your max memory setting?
Is this a VM or a physical box?
Jason AKA CirqueDeSQLeilI have given a name to my pain...
MCM SQL Server, MVP
Posting Performance Based Questions - Gail Shaw