Flight from Istanbul
It triggers if the Rumelia region (Just west of Istanbul) and East Macedonia are not owned by The Ottoman Empire. It does, of course, also require that The Ottoman Empire does in fact own Ankara.
It can happen if either Bulgaria gains independence, or Greece takes the decision The Megali Idea and successfully acts on it. It is needed in order to form Byzantine Empire, as Istanbul is a required province and it cannot be annexed while it is the capital.