Propaganda for their faith. In 1451 King Casimir recognized Met. St. Jonas' jurisdiction over all the Orthodox in the Commonwealth except the diocese of Halych and Peremyshl/Przemyshl, whose bishop a century and a half latter refused to sign the capitulation of Brest and help lead the opposition to it.
I want to know more. I am unwilling just to dismiss a specific assertion of a date out of hand as "propaganda." What happened in 1416, the date when the Catholics allege that the eparchy of Przemysl went over to the "schismatic" Orthodox?
I know that Przemysl did not entirely accept the Union of Brest until 1692. My ancestors resided within its territory. It had parallel hierarchies from 1610, however, and the Orthodox bishop had his residence in Przemysl, with a coadjutor at Sanok. The Greek Catholic bishop had his seat in Sambor.
EDIT: Fixed the locations of the bishoprics, as near as I can tell.