Half and half

We've been watching this backyard tree transform from summer into fall. The leaves are changing but, not all at the same time. Half the leaves are almost completely changed while the other half remains green.

I'm sure there's a reason but, we're not quite sure. I guess the logical reason might be that the sun hits the changing side first each morning thereby protecting the green side?

Any tree experts out there care to clarify? Also, not sure what species this tree may be. Talk about horticulturally challenged!