WCF : Callback reentrancy is not clear. Why this dead lock take place

I was reading an article by Juval Lowy. Below section is not clear to me.My Question is : Why callback re requires the ownership of lock? I mean the thread in which the call back is called is waiting for callback to return.so this thread should resume after the callback returns, just like any other method call.
 
The service may also want to invoke the callback reference passed in or invoke the list of callbacks during the execution of a contract operation. However, such invocations are disallowed because by default the service class is configured for single-threaded access: the service instance is associated with a lock, and only one thread at a time can own the lock and access the service instance. Calling out to the clients during an operation requires blocking the service thread while invoking the callbacks. The problem is that processing the reply message from the client once the callback returns requires ownership of the same lock, and so a deadlock would occur.
To avoid a deadlock, if the single-threaded service instance tries to call back to its clients, Windows Communication Foundation will throw an InvalidOperationException