I wasn't thinking about religious differences as much as the physical assault against a church leader which is itself an act of war. Though in this case it looks like local security, so there may have been a peasant revolt against the local lord for treachery. I mean you had hundreds of battles over much sillier things. (popes engaging in territorial conquest, land squabbles between barons, drunken celebration marches turning into battles, and the "War of the Oaken Bucket" which probably wasn't really over a bucket but was still pointless tit-for-tat for no good reason) If you're requiring the Church to define "just cause" then yes that would depend on what the Orthodox church officials said.