"If some one has a logical answer and some reference, please come up."

It's in The Book already mentioned.

The "80C51 family hardware description" comtains a full description of the detailed hardware operation of the UART; and yes, once you've put the data into the SBUF, transmission proceeds autonomously to completion.

If you think about it, it'd be unusable if it didn't - you can't pause transmission of an asynchronous character part way through!