If I remember correctly LibreOffice split off from OpenOffice because they wanted to focus on more features while OpenOffice wanted to work on stabilizing and feature compatibility.
I agree, OpenOffice should merge back with LibreOffice if they can.
Not quite. LO split off because of fears over the future of the management and the licensing of OO. Oracle bought them and was not treating the project well. There was real fear that the OO project was going to be shut down and the code removed from circulation. LO was formed to protect against that.
OO and LO then additionally had some different opinions on development direction but that was purely in addition to the major issues that caused them to split.