That's technically a sectorial chimera.
The reason this happens is you have two genetically different cells in the apical meristem, with each contribuiting daughter cells for one half of the leaf. Apical meristem is the growth tissue found at the tip of the roots or, as in this case new shoots and daughter cells are exact clones of the original cell.
As for the molecular mechanisms underlying a red leaf (technically it's called a stable leaf phenotype), that's usually due to high activity levels of activity involved in the flavonoid-anthocyanin synthesis pathways through all leaf development stages to maintain a steady flow of anthocyanins. Forgive me if I do not use 100% correct terms but I studied this stuff what feels like a couple centuries ago.
What happened is one of those two cells of above was a mutant, or to be more specific it was a mutant of a mutant, expressing low levels of activity in the aforementioned synthesis pathways, hence allowing one half of the leaf to look green.
As per why it happened, absolutely no clue.
