March 25 is like many other Orthodox feasts, where the spiritual and historical, ecclesiastical and civic aspects of Orthodox life meet in symphony. We celebrate freedom from the tyranny of sin, brought about by the Incarnation of our Lord, and freedom from the tyranny of Muslim oppression, brought about by a revolution blessed and maintained by the Orthodox Church.
This is nothing new. We have many such feasts. The Akathist Hymn itself was created to celebrate the Empire's defeat of the Avars in the 7th century. That's an entire category of liturgical worship, and a good number of feasts, that have their origin in a "secular" independence story (although there is no such thing as "secular" in the Byzantine mind). There are other such events in the ecclesiastical calendar, ranging from battles, to earthquakes, to fires. We commemorate the losses and celebrate the victories.
Such is also true in the other Orthodox churches. There are too many to name, but the biggest include the Battle of Kosovo (producing St Lazar the Great Martyr of Kosovo) and Stephen the Great in Romania (Ștefan cel Mare și Sfânt).
Theologically, this presumes a firm belief in God's providence. History, culture, civil society, etc. are not separate from the Church, since Christ is King of all. That's a very different way of thinking than the liberal, French-inspired point of view prevalent in the American popular psyche, so it takes some time and experience to understand.