I presume the kontakion in question is "Ti Ipermaho", translated I believe as "Triumphant Leader" with that translation. This is one of the kontakia used in the Akathist hymn. It is said that it was added to the Akathist by the Church of Constantinople in the 600s after a storm destroyed an invading enemy, saving the city. This was attributed to the Mother of God because of a procession of an icon during the city's defense.
The kontakion begins by proclaiming a feast of victory to the super-powerful, victorious general (very rough paraphrase of one way it could be translated - this is actually very hard to make a good English translation and of the 20 I've seen I don't think any have done it really well). Her power is, of course, Christ, who was incarnate through her, by whom all things can be done, and to whom she prays for the Church.
Finally, why were they singing it in October? I can think of no reason for it, unless it's the parish's kontakion or they're having an Akathist.