I wonder what is ethnogenesis of this people.

They are just one of the shards of collapse of Kievan Rus who had lived under foreign rule for too long separated from the other parts of Rus whose Western Parts after Mongolian invasion became firstly the parts of Grand Ducht of Lithuania and the – of the Reczpospolita, were subsequentlay called in Latin way – Ruthenia, and whose Eastern Part was subsequently named in Greek way – Rossiya.
And all of them were called Ruses, Russins or Russichi. Rusyns have the same origin as all of us but have developed into a separate ethnic community ever since. Ukraine does not officially recognise Rusins as a separate nation but does not presses the point. The general opinion about them is that they are Ukrainians living abroad.