It was forked from OpenOffice after Oracle bought OO, and most of the core developers moved to LibreOffice
https://en.wikipedia.org/wiki/LibreOffice
LibreOffice is a continuation of the original development team, and the original code. The only thing they don't have is the name, which doesn't matter much in my mind. So why do you say that what's now called "OpenOffice" is the main line, and "LibreOffice" is a fork? What's your definition of a fork, exactly?