Most of the Information used in this is taken from fr Peter Farrington's Podcasts on this issue, which are found here.
So, why and how does the Tome condemn the Church fathers, and what would lead the Monk Samuel to react in such a disgraced manner when asked to acept it? The answer is quite simple; the Tome of Leo states that "It is just as impious to say that the only-begotten Son of God is from two natures before the incarnation as it is unlawful to assert that after the Word became flesh there is one nature in him” In this, it Anathematises those who speak of 'One Nature,' which was the basis of the Cyrilline Miaphysitic Christology and the Alexandrian School.
The worry of this is picked up by Pope Dioscorus of Alexandria at the Council of Chalcedon. He, after his deposition for being a followe of the Cyrilline Formula, Anathematised the council of Chalcedon in 5 sections for various issues which were not addressed. Dioscoros' second Anathema stated that "Chalcedon is anathematised because it has trampled under foot the canons and prescriptions of the Fathers." Bringing up wholey Orthodox Concerns, since the use of "two natures" was feared to be (and is still seen as) using the language of Theodore and Diodore, both who used a Christology incompatable with Cyril's.
Though these concerns were not seen as Heretical by the Chalcedonian Church, they still refused to restore or remove the Anathema on the ‘One Nature’ teaching of Dioscorus. His concerns were in fact addressed late, with the Patriarchs who had opposed Chalcedon and raised these very concerns not being invited to attendance.
Due to the confusion and Theological Protest raised by the Chalcedonian condemnation of Cyrilline Christology it took another 'Ecumenical' Council to fix the errors and misinterpretations of Leo's Tome which arose. One of the Canons from the 5th Council stated that "if anyone shall calumniate the holy Council of Chalcedon, pretending that it made use of this expression [one hypostasis] in this impious sense, and if he will not recognize rather that the Word of God is united with the flesh hypostatically, and that therefore there is but one hypostasis or one only Person, and that the holy Council of Chalcedon has professed in this sense the one Person of our Lord Jesus Christ: let him be anathema."
Regardless of the 5th Council placing Anathema on all who interpreted The Tome of Leo as using the language of Nestorianism, many EO still call OO Heretical for bringing this fear up at the council. Dioscorus was in fact Anathematised for trying to raise these concerns. So why was he so fervent to fight the language of the Tome?
By Anathematising those who refer to the incarnation as "one nature," the Tome Anathematises Athanasius and Cyril, both great thinkers of the Alexandrian School of Christology and Orthodox Church. When speaking of Christ, Saint Athanasius stated that "there is one nature of the incarnate Logos" emphasising that "Jesus isn't two natures , we kneel to one without the other , but one nature is the incarnate Logos, kneeled to him with his body one kneel"
Saint Cyril is the key thinker here. being the great defender of Orthodoxy and man who drove Nestorius out of the Church he is certainly not a Heretic, yet by the Tome of Leo, he is. As shown here, The writings of Saint Cyril also speak of one nature, saying that "we say that the two natures united and from them one Lord and one Son is resulted as we agree by our thought but after the unification the separation to two natures is removed, we believe that there is one nature the Son as One person, one humanized and incarnated"
Cyril and the Council at Ephesus even Anathematised those who speak of Two Natures, saying “If anyone distributes between the two persons or hypostases the expressions used ... and ascribes some to him as to a man, thought of separately from the Word from God, and others, as befitting God, to him as to the Word from God the Father, let him be anathema.” So to reject the Cyrilline Formula of One Nature after the Incarnation was Anathema after the Council of Ephesus, yet to accept it was Anathema after Chalcedon. This cimply brought confusion.
In conclusion, The Oriental Orthodox Rejection of Chalcedon is not based on support for a Heresy but for support for the Church Fathers who taught the same language as those the council deposed and anathematised as heretics.
After the Council had passed this judgement on all who support the idea of one Nature, it caused a Schism which has lasted over 1500 years. In that time, the non-Chalcedonians have called the Chalcedonians Nestorian for their rejection of Cyril’s Formula and the Chalceconians have called the Non-Chalcedonians Monophysite in return. Both are misinterpretations of two clear Christological Positions.
This issue has never been fully addressed and seemingly never will. Chalcedon’s premature condemnation of those following the Alexandrian School of Christology in favour of the Chalcedonian Formula needs to be addressed . The Anathematising of a teaching followed by the Fathers has never been explained, only responded to with accusation of Heresy.
There is a justification for the Oriental Orthodox Rejection of Chalcedon and it is an Orthodox one. As Father Peter Farrington explains, these issues are important, as “reconciliation of the Chalcedonian and anti-Chalcedonian communions still demands that they are treated seriously and eirenically by the Chalcedonians. They are often brushed aside as irrelevant, but a proper understanding of our own tradition requires that they answered.”