The majority are in communion.
But it does happen for various reasons why certain bishops will separate themselves (I like to call it mini-schisms). These are often over issues such as ecumenism, beards, the calender, or some various practice within the church that changed.
Often it is a battle between the bishops, who have every right to schism if they don't feel something is right.
Sometimes some bishops would consider themselves in communion with another Orthodox branch, but that Orthodox branch would not be in communion with them.
Often these schisms center around traditionalist types of things.
If there are two bishops who agree and break away from an orthodox "bunch", then they are self sustaining because they can ordain more bishops.
At one time, the Patriarch of Rome (pope) and the rest of EO were in communion, and that schism was "the Great Schism" (1054 A.D.). That dealt mainly with Papal (Rome) supremacy and the filioque added to the Ancient creed.
There have been many reunification attempts, but the Pope won't eradicate the last 1000 years of RC stuff, remove the filioque, and become and "equal" with the other ecumenical patriarchs.
So sometimes the EO churches go back into communion, and sometimes they never do.