I don't consider Chalcedon to be heretical, but it's easy to see, in the context, how an orthodox Christian, living at the time and loyal to St Cyril, could find it to be highly suspect at least.
1. The language of the definition of Chalcedon was new. St Cyril's famous formula, "One incarnate nature of God the word" was well-known among Greek-speaking Christians; "One person in two natures" sounded to many like a compromise with Nestorianism. For those grounded in Aristotle's metaphysics, as the Alexandrian school very much was, the idea of an abstract nature distinct from a person didn't make sense- "physis" and "hypostasis" were used more or less interchangeably. The school of Theodore of Mopsuestia also said "two natures" but meant something quite heretical, the natures being united only by common will. The closest thing to the definition of Chalcedon (and the Tome of Leo) was the formula of union reached between Cyril and John of Antioch, which used "of two natures" (not "in two natures"). So the fathers at Chalcedon had some explaining to do, as to why they were introducing this novel formula, and how it could be reconciled with the previous language- explaining which evidently they did not bother to provide. This task would be left to the next council.
2. The rehabilitation of some followers of Theodore and Nestorius was also a red flag. Ibas' letter denouncing St Cyril was read at the council and declared orthodox (this same letter was condemned as heretical at Constantinople II). Theodoret was likewise reinstated and played a role in the subsequent proceedings. Both these men where required to anathematize Nestorius, but this was likely conditional and insincere ("if he really did teach x, y, z, then I denounce him") and they did not retract their condemnations of Cyril.