Nintz
King of the Britons
This is part of the 4th Crusade dynamics. If you play as a Catholic player and you get a lot of gold, then you can "buy" a Crusade, becoming the leader and sending it towards the Capital of any player. The AI will only do this as Venice -> Constantinople and Spain -> Cordoba.
Okay, in my recent game as France, which I've almost won, I've seen Venice do this twice. I also saw Genoa target Constantinople once though. Note: By now my French Empire had vassalized Venice. Does that affect the Crusade mechanic?