Вы должны прочитать AsyncSocket.h, следующее очень близко к вершине (первый обратный вызов фактически даст вам информацию о причине разъединения, которая может быть полезна)
/**
* In the event of an error, the socket is closed.
* You may call "unreadData" during this call-back to get the last bit of data off the socket.
* When connecting, this delegate method may be called
* before"onSocket:didAcceptNewSocket:" or "onSocket:didConnectToHost:".
**/
- (void)onSocket:(AsyncSocket *)sock willDisconnectWithError:(NSError *)err;
/**
* Called when a socket disconnects with or without error. If you want to release a socket after it disconnects,
* do so here. It is not safe to do that during "onSocket:willDisconnectWithError:".
*
* If you call the disconnect method, and the socket wasn't already disconnected,
* this delegate method will be called before the disconnect method returns.
**/
- (void)onSocketDidDisconnect:(AsyncSocket *)sock;