While it is true the CCEO does not mention the title catholicos, the Armenian and Chaldean Catholic primates have traditionally used the double title of patriarch-catholicos. Also a motu propio issued in 1969 states:
30. With regard to the dress and titles of Cardinals and Patriarchs of the Oriental Rite, the traditional usages of those Rites shall be followed.
All laws that apply to patriarchs also apply to major archbishops. Catholic usage has equated catholicos with patirach and major archbishop. Once promoted to major archbishop, the Malankara Catholic metropolitan saw it as natural to assume the title of catholicos as a traditional usage.
Thanks, Dn Lance.
To be very precise, the "traditional" usage of the Church in India for its primate is "Metropolitan (and Gate of All India)" or "Metropolitan of Malankara", with other bishops technically being his suffragans. When the Orthodox Church in India split into autocephalous and non-autocephalous jurisdictions, the former assumed/was given the title of Catholicos of the East, the title of the Orthodox primate of the Church of the East, based in Persia (another Thomasine Church); the title had fallen into disuse since the Persian Church was basically "Nestorian" and not Orthodox. The Catholicos was the head of the entire Orthodox Church of the East, Persian and Indian, while the Metropolitan was the local primate of the Indian Church. Originally these were two distinct offices held by two different bishops, but eventually both titles were vested in one bishop (though it could, theoretically, be separated again). To my knowledge, the non-autocephalous jurisdiction never had Catholicoi until the 1970's when, after a reunion of twelve or so years, the schism resumed, and the Patriarch of Antioch gave the title to one of their bishops. Though at first the title of their primate was also "Catholicos of the East", the current incumbent is officially styled "Catholicos of India", and does not claim the office of "Metropolitan of Malankara" or "Successor of St Thomas"--only the autocephalous primate does so.
In light of this, it's not really a "traditional usage" of the Indian Church to refer to the head of the Church there as Catholicos, but only as Metropolitan. The Catholicos is really the head of the Orthodox Church of the East, the heir of which neither the "Jacobite Church" in India nor the Syro-Malankara Catholic Church claim to be. From this perspective, it doesn't really make sense for the Major Archbishop to claim the title Catholicos, even if they don't claim explicitly the throne of the East (which, to their credit, they don't): technically, it's an innovation. It makes more sense, IMO, when seen against the background of the sheep-stealing that the Malankara Catholics are prone to doing.
This is confirmed by the other title the Major Archbishop has assumed: "Successor of the Apostolic See of St Thomas". Though St Thomas ordained bishops and priests during his missionary activities in India, the real "lineage" that this title is traced from is the Catholicosate of the East, also founded by St Thomas. Unless I'm mistaken, "Successor of St Thomas" only really entered India with the Catholicosate (of the autocephalous Church), and was thus a matter of contention between Antioch and India. Since the Major Archbishop is not the Catholicos "of the East", "Successor of the Apostolic See of St Thomas" doesn't make much sense either: I don't believe the Metropolitans of Malankara used such a title. So again, it doesn't really make sense (to my knowledge, no other Eastern Catholic primate claims to be the successor of a particular apostle, even if his "Orthodox" counterpart does, though you may correct me on that).
The only title the Major Archbishop could assume with any sort of historical tradition behind it is "Metropolitan of Malankara", and he has. He's the only primate of any Church in India to claim that title other than ours (and it was from our Church that Mar Ivanios broke away). It's a gutsy move, but it's not like there's a major push to return to traditional, Orthodox practice at all levels in Malankara Catholicism. There's still self-imposed mandatory celibacy, Latinizations like the use of unleavened bread, mandatory introduction of Western devotions, religious orders, etc. Why is it that, at the level of primatial administration, they are so eager to "be Orthodox" but at other levels they are happy to be a hybrid? I'm sorry to say it, but it's all part of the sheep-stealing IMO. They see the internal squabbles between the Orthodox and take advantage of the situation to attract people to their parishes. Moreover, their association with Rome gives them more influence than they'd normally have with their numbers, as they have access to resources we don't, and are not afraid to use them. They use those resources and connections to do a lot of good work, mind you, so it's not like I don't support them in the good they do (because they have their act together and we're busy shooting ourselves in what remains of our feet). But when Rome tells us that the "Uniate" model (as opposed to Eastern Catholicism) is a thing of the past, and then the Malankara Catholics go off "Uniating", when Rome tells us the Major Archbishop is just a major archbishop and does not have the titles he says he has because they don't exist in canon law, but lets the Major Archbishop continue claiming them, it just seems like a lot of duplicity.
Anyway, the more the merrier, I guess.