bot has not received any response from the server in a certain period of time (eg, server is lagged, or bot is lagged, or server has completly vanished)
This usualy happens when a server splits (or when the server sends non-RFC complients messages).
Due to the high internal traffic a server receives and processes during splits, messages sent from the bot to the server, take a long time to be processed (are you quick to answer when stonned?).
If a server is still alive, but isn't answering messages, then this message is received.