I think this is simply the choice of the color scheme that makes this plot the way it does. People often "cheat" with that, and here it the data seems to be clustered both by rows and columns with a custom color scheme. Visually there does not seem to be a reason that would justify the red lining up so well along that diagonal, but I think this is simply the cells over/under a certain value are simply plotted as the same color of green whereas there may be more variation to them.

It is a common seen (some would say misleading) choice to limit the color-scheme in a way that leads to a visually striking pattern.