Why does foliage coloration vary on north/south sides of ridge (Northeast USA)?
Your photo doesn't show the strip of autumn-coloured trees very well; without your description I would have taken it for a strip of bare earth. All sorts of things could account for the autumn shades, there are too many unknowns to give a definite answer.       We don't know whether the red-leaved trees are the same species as the others, for example.

I doubt if sun exposure alone could account for it, though it might be a combination of sun exposure and soil conditions, or soil conditions alone. The red trees seem to follow a line where strata different from the rest reach the surface. This would affect the soil conditions, nutrients and perhaps available moisture. Without an investigation on the ground it is impossible to be sure what the cause is, but I think a difference in soil conditions is the most likely explanation.