I wonder what is ethnogenesis of this people.
In my opinion, not very educated one regarding them, I believe that they are actually original Ruthenian people, recognized as such by Polish-Lithuanian Commonwealth at time when Mongol Empire fell apart and those found on Western side (Polish) became known as Ruthenian while those on Eastern side (Russian) became known as Muscovites.
Then over time, Ruthenians created two separate nations: Belarus and Ukraine. I wonder if those Rusyns are those who did not choose on of this nations to belong to, and stuck with their roots. Are they ones who keep legacy of Kievan Rus alive and are those who identify with this old ethinicity, who gave birth to all East Slavs?