Flight from Istanbul is an event that can occur to the 
Ottoman Empire (Turkey), that changes the capital from Istanbul to Ankara.
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.
There is only one possible option We have been ultimately driven from Europe.., which removes 10 prestige and moves the Capital of the Ottoman Empire to 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.