The explanation is that the user is running an inferior POP3 client that spawns multiple simultaneous POP3 connections. The POP3 protocol permits only one POP3 connection at a time. Also, the traditional UNIX mailbox format only can have one read/write access at a time. POP3 is always read/write.
The message simply indicates that the new POP3 server session (in your example process id 27997) grabbed the mailbox lock from the old session (process id 27995) and killed the old session. That's why the old session wasn't there when you looked. The closeness of the process ids indicates that the sessions were created in fairly rapid succession. This is not a problem in ipop3d. The fix is to have your users use quality POP3 client software that complies with the POP3 protocol. You should be concerned that you have users who use harmful software that wastes system resources by spawning multiple simultaneous POP3 sessions.
